Rear End Swap

[tjg_26]

Greetings everyone, I have a 1971 Ford F-100. I put in a 390 out of a 1978 F-150 (yes in 78' they had 390's in commercial trucks). The guy i bought the truck from had a little 302 and a car rear end in for speed, not power. Now I have the rear end from the 78' still. would it be better to have a new driveshaft made, or swap the rear end?

Thanks, Tyler

[FORDification]

That rearend won't fit. The spring pads are farther apart on those truck's rearends. The third member will fit your existing housing, but it's probably set up for 31-spline axles (as opposed to 28-spline axles in yours now), and the longer axles won't fit your current housing.

You'd either have to move the spring pads to get that rearend assembly in there, or try to find some fairly-rare 31-spline axles that'll fit your existing narrower housing.

[pat67shorty]

How much wider are the 73-79 rear ends than the 67-72?

[FORDification]

On the '67-'72 trucks the rear track width is narrower than the front....that is, the rear wheels don't track directly behind the front. In '73 when the rear frame rails on most trucks was widened out 4", the rearends also became 4" wider, which brings the rear wheels out to track directly behind the fronts.

[binder56jd]

what was the reason behind the narrow rear?

[SmokeyBear]

what was the reason behind the narrow rear?

They aren't married yet.




























Historically older vehicles just had a narrower frame. As technology caught up the frames got wider to stabilize the vehicle.

[ToughOldFord]

[binder56jd]

funny guy i didn't see that one coming

[tjg_26]

thanks everyone. I have a different question now. Are the driveshafts the same for stock rearend 1971 and 1978 engine/tranny? they are both half ton trucks, and are both 8 foot box trucks.

If so, I am going to the junkyard and pull the rearend and driveshaft out of a early 70's truck.

Thanks,
Tyler

[fordman]

the 78 had a one piece drive shaft as where the long beds of 67-72 had a two piece driveshaft.