Bob Westphal's P-15 Slant 6 Conversion, with 904 transmission

By Bob Westphal

Scroll down the page to see Bob's pictures of the process.

P-15 & YvonneWhen I became interested in this project, I received lots of negative feedback and advice as this conversion is almost unheard of. Below are the problems involved along with my comments on each.

The slant 6 is longer and wider than a MOPAR 218/230 flathead engine. Will it fit in the P-15 engine bay?

Yes. The SL6 is 25” long, which is the same length as the 251ci MOPAR 6. This is a common conversion in P-15s and is handled by moving the radiator to the front of the support frame. The only modification is notching the aft portion of the support at the bottom to gain access for the fan belt. The cross member that, was used for the rear engine mount, needs to be removed. For width concerns see below.

Can the stock steering, suspension, rear axle, and brakes be used?

Yes, on everything except the rear axle. The driveshaft and rear axle aren’t compatible with the modern components. The ‘B’ body rear axle is a bolt in for the P-15. The ‘B’ body donor driveshaft had to be shortened 1”. You will need to fabricate headers when using the stock steering, as headers on the market won’t fit because of lack of clearances between the steering gear and exhaust manifold.

Will there need to be body modifications?
No. The engine fits nicely between the stock inner fenders and clears the firewall. However, if a 904 gear box is used, the removable floorboard will have to be clearanced due to the girth of the 904.

What about shifting the gear box?
The stock shift lever is used with the 904 using modified stock linkage parts (See photos). With a four speed, the floorboards will need to be cut to accommodate an aftermarket floor shifter.

Will the alternator need to be relocated to other side of engine?
No. It fits nicely over the left side inner fender shelf when used in the stock position. You will probably need a shorter belt, though.

Will a rear-sump oil pan and pickup need to be used if stock suspension and steering components are in the way of standard, center sump pan?
Yes! Slant 6 vans and trucks are the source of these parts.

What about engine mounts, tranny mount, and cross member?
Of course engine mounts will have to be fabricated as well as a cross member/tranny mount. I used the donor car’s stock motor mounts bolted to the engine. I fabricated frame mounts from 3/16” X 2” angle stock, welded to the frame rails. The right side engine mount was welded to the underside of the frame rail where the left side was welded to the top of the rail. The engine was tilted toward the left side 6 degrees to allow clearances of the right side firewall and the steering box. The cross member/tranny mount assembly is also fastened to the frame lower on the left side than the right (See photos). I fabricated two pieces of 2” square tubing welded to 3/16” steel plate on one end, bolted to each side of the right and left inner frame rails. The stock tranny mount was bolted in between tubing assembly (See photos).

In summary, this was a fairly easy conversion and much easier than a V8 conversion, which requires altering the inner fender sheet metal. Like all conversions there are always small hurdles to jump over. The exhaust system presented a problem since the stock manifold and the steering box are too close in proximity for header pipes to fit in the small space. I fabricated a header setup with outlets that fit in front of the motor mount and the starter respectively. I used most of my original dual exhaust system. The clutch pedal was cut off at the base. I used the original radiator hoses by cutting them to the required lengths. I also used the original bulb type temperature gauge by hooking the bulb into the heater hose. There is actually room between the engine and the radiator for a fan. I used a 13” diameter flex fan in conjunction with an electric fan in front of the radiator.

The shift linkage was quite simple. I used the stock shift arm on the column by turning it to stick straight up and bolting another ‘L’ shaped piece cut from 3/16” flat stock. The original longest shift rod was inverted with the threaded end bent and cut to fit into the tranny’s shift arm (See photos). The 904 gear box has built in neutral safety switch that is wired to the starter solenoid from the donor car. I mounted an aftermarket transmission cooler mounted at a forward tilt under the car next to the transmission.

I used an aftermarket throttle cable assembly with a gas pedal robbed from an old Duster and bolted to a fabricated mount on the inside firewall (See photos). Since I used headers, I am using a manual choke.

Click on any of these images to open a larger version in a separate window.
1244 1246
Hole that needs filled
with SL-6
Trial fit
Tech.Committee at work
Firewall clearance
1242 1243 1258 1260
Clearance pretty tight between exhaust & steering gear
Position of crank pulley requires removal of metal to allow clearance
Fabricated half of rear X member- 2” tube welded to 3/16th plate
Donor stock tranny mount
Note donor tranny mount is turned 180 degrees from stock position on tranny to keep X member inside frame rails
X member installed donor tranny mount incorporated in center of member
Right side engine mount located near bottom of frame aft of lower right
‘A’ arm
Left engine mount welded in place from topside
Look at bottom of left mount
Radiator support relieved to allow fan belt installation on crank pulley
Engine installed showing clearanced radiator support
Engine fits tight but looks good in place
Clearance between radiator and water pump allows space
for 13” flex fan used with pusher electric fan. Also shows
placement of radiator mounting
Installed engine showing stock donor alternator location
Modified original shift rod with fabricated ‘L’ arm
Modified original shift linkage keeping original shift mechanism
Home made headers
Completed installation; note header location
Another look at alternator location
Crude but effective stock bulb temperature location
Fabricated gas pedal mount bracket using Duster
pedal assembly.
Fabricated gas pedal mount bracket using Duster
pedal assembly.