73-74 to 78 frame and rear differences.

[chevymad]

I've got 3 trucks that i'm putting together to make 1. The first one is a 73 C10 original 454/th400 truck. Bought the truck cheap to get the engine/tranny for another project. Then my brother gave me a rusted out 74 3/4 ton, good running gear-no motor or tranny. Then I inherited a 78 crewcab, again rusted out, but has a 91 crate and working th400.

Anyway, after stealing the 454 out of the 73, I realized I still had enough parts to make 1 decent 3/4 ton, 350 powered truck. So I took the front end,crossmember and all, out of the 74 and installed it in the 73. This is when I found out the 73 1/2 ton 454 frame is 3/4" TALLER then the 74 3/4 ton frame. The 74 and 78 3/4 tons are both the same. Weird I expected the 1/2 ton to have the lighter frame. So I had to use the 1/2 ton lower engine frame mounts to make everything work.

Now today I go out to put the 78 14bolt rear into the 73. I pull the old 12 bolt out, install the exhaust while everythings open and easy. Then roll the 14 bolt springs and all under the truck.. ACK! It's a couple inches too NARROW! I measure across the leaf springs and the 78 3/4 ton measures 42 1/2" the 73 1/2 ton measures a hair over 45"! Curious I go out and measure the rear under the 74. It's the same as the 73 1/2. Looks like i'll be pulling that rear out now.

Measuring the width of the frames all are the same, the difference is in the mounting brackets. I could cut all the rivets off and change brackets, but since I have a rear that will fit, I wont go that way.

Always thought these 73-80 trucks were nearly the same, but theres alot of differences that dont show.

[bigblock73]

That is strange...I never knew they were different either. It would actually be cool to find out all the differences thruought the years and have some form of article on that. But unfortunately since the frames are rarely seen, and most people don't have numerous bare frames laying around, not too many people care or have time for that kind of stuff.

Thanks for the info you provided, I am going to keep notes of this.

[Captkaos]

The 12bolt came out of the 1/2T truck and the 14Bolt came out of a 3/4T crew cab truck. The 14bolt mounting is narrower because of the dual wheel option. What is the measurement for the rear diff on the 74 3/4T? It should be the same.

[chevymad]

All rears are single wheel. The rear on the 74 3/4 ton is the same width as the 1/2 ton. It's the 78 3/4 ton thats narrower, only the springs themselves are mounted inboard further then on the other rears.