I find it hard to believe ford made different frames for different models. It would not be cost effective to have different tooling and such to make a lighter frame. Maybe a few of the frame components are different, such as spring hangers and cross members. I would think that all regular cabs from f100-350 would have the same main frame rails, same for other configurations of cab and box length.

I could be wrong but I just don't see how they could have justified it.

 

While the frames looked the same, they indeed had different strengths, refered to as "frame modulus".
Here are some pages from the Ford parts catalog showing the different frames and part numbers, there are a BUNCH of them. It also shows the different frame modulus of the frames.

 

Well I guess I learned something new, kinda why I come here though.

 

Yeah, I can tell you just from going to an F150 reg cab 4wd to an F250 reg cab, you are gonna see different frames. 2wd's had differnet frames from 4wd's. The supercabs and crew cabs had different frames than the reg cabs (thickness and height as well). And the F100's had different frames. There may be some instances where they used the same frames (perhaps a 4wd F100 supercab to a 4wd F150 supercab, or something) but they used a baffling variety of changes that you would think would have been standardized.
Its no surprise the frames were different from the half ton frames to the 3/4 and heavier frames, as those trucks were rated a good bit heavier, used different axles, different springs, different front suspension type (leafs), different steering boxes.

Same bodies for the most part, but underneath they are very different trucks.

 

x2 what husker77 said. Thanks for the education. never disappointed when I log-on FTE. My wife gives me grief for the amount of time I spend reading threads on this site before I attempt anything new on my rig. She thinks I'd be done with most of the work already if I just grabbed a wrench and went at it. I keep telling her I'm saving us time and $$$!

 

Any idea if the bed off a 76 will fit on a 78 f150?

 

Interesting... my '79 F-100 2wd has 6 leaves on one side, 7 on the other!

 

+1 on Mike's post - I was working for a Ford dealership back then and his explanation is dead on. They found that by making the F100 frame closer to an 3/4 ton frame they could squeeze the weight just above the 6000 pound lower limit for non-emission controlled vehicles. At the time they said "putting a 3/4 ton frame under the half ton truck" but they really did not do that, just swapped over some heavier components to get the weight up to (as I recall 40 years later!) 6050 pounds or so.

 

i have seen one f100 with a 31 spline, with that been said how would i check mine to see what it is/ an easy way would be preferable.

 

The only way to truly know is pull an axle and count the splines.

 

Yes it will, with a couple of "ifs".
If the '76 is from a F250 4WD (AKA "Highboy") you'll need to redrill the mounting bolt holes. F250 4WD through mid-77 (s/n Y20,001 to be exact) used a narrow frame. 2WD and post mid-77 4WD trucks used a wide frame.
The second "if" is appearance related. The '76 has an exposed fuel filler cap, the '78 has the fuel filler behind a door.

 

I have picked up two 1976 regular cab trucks--both in really rough shape, but all original. One is a F100 and the other is an F150 so after reading this thread, I will definitely be crawling around under them noting some of the differences. I do know that the F100 definitely has 31 spline axles and there is nothing "special" about this truck. It is a custom cab, 302, with a T18 4-speed.

One thing that really hasn't been discussed is engines used in F100's versus F150's. Now I haven't researched this for complete accuracy so please bear with me and feel free to chime in on this. I looked at the dealer brochures for a few years and the patterns change slightly from year to year. Here are a few examples of engine applications for the F100/F150 2 wheel drive trucks (I have deliberately excluded F250 & F350):

300 was available in the F100 & F150
302 was used in F100 but was shown available in F150 in 1977.
360 was used in F100 & F150
390-2V was only used in F100 in 1975 & 1976
390-4V was only used in F150 in 1975 & 1976
351M was available in both the F100 & F150
400 was only used in F150 in 1979
460 was only available in F150

There does appear to be a pattern where the larger/higher horsepower V-8's were used in the F150's with some exceptions. The 300 was used in all trucks which makes some sense because of the 6 cylinder's "workhorse" torque characteristic.

 

My 1977 F-150 has a 400 small block

 

Yes, the 400 was available in both the F100 & F150 in 1977 & 1978. F150 only in 1979.

 

Fifty (50). .