i took the bed off my truck (78 body on 76ish highboy frame) and found my frame is shot. my cousin has an 84 6.9l with a destroyed body. both trucks are standard cad 3/4 ton 4x4. how much modification would it take to put my body on the 84 frame

 

I can't be for sure, but I believe the major mount of the modification would be the cab and core support mounts. The wheels bases should be the same. Best thing to do would be to take some measurements and go from there. Other than maybe needed to drill a couple new holes, the bed should mount right up to the 84 frame.

 

would the 6.9 have any problem fitting in the engin compartment or the tranny have a problem going under the floor of the cab

 

you shouldn't have much problem fitting the engine in the compartment and the transmission should fit, but there again depending how you would have to move around the cab and body to fit the newer frame.

 

This won't answer your main question but hopefully it will help a little.

Having just put an '87 (pretty positive it was an 87) bed on my '74 F-100 supercab 2wd I can tell you the bolt holes in the beds aren't exactly the same like I had read...

From what I had heard/read the inside of the beds were the same from 73-96 - aside from the holes being in a different spot for the 4wd trucks that had the narrow frame rails.
While I had a bed on my '74 I was going to be cutting the stock bedsides off and replacing them with flared fiberglass bedsides. A guy had asked me if I could sell my bed and after thinking about it for a bit and reading the 80's and 90's beds would "bolt on" I told him if he gave me some cash and a replacement bed in decent shape I would. Since 80's and 90's beds are easier to find and cheaper this worked for him.

Well I got the 80's bed on and there are some differences. For starters the 87(?) bed I got uses 6 bolts to hold it on instead of the 8 my '74 had. Granted you can see where the 2 other bolts would/should go and there is a hole in the bottom of the bed - but it doesn't come up through the top layer of the bed skin.
I have the 2 rearmost bolts in the bed through the frame and the other 4 from the 80's bed are slightly off from the holes in the frame on the '74. I didn't spend a ton of time looking at it as I was just setting the bed on there for now, but it seems as if the holes in the frame on the '74 are about 1/2" or so outside of the holes in the bed. They are close, but not close enough to get the bolts to go through.

So the shortened version is - the beds are close and will swap but I'd expect to drill some holes.

Here's pics of the bed/truck I was referring to.

 

the cab souldnt require much other than moving the cab mounts should it?

 

I put the body off my 77 onto the frame of an 89 f350 with the 7.3, same dimensions as the 6.9 which it sounds like you would be using from your description. The front cab mounts need to be moved forward and up to get the body to fit. What I did was just build on the existing mounts connected to the frame. The rears are easy after that, just have to move them the same. The bed holes did not line up at all as the frames were a different width. I needed to make spacers to line the bed up with the body, after I had those it was just drilling need holes in the bed. Its not all that difficult, but it will take a lot of work to get it done. The rad support was also a pain, since it needed to be modified to accept the larger radiator and fit on the new frame.

 

do you have any pics of your build

 

Ive got a few. Here's a couple that give a basic outline
Here is the 350 frame before I started work on it

And having to extend it to accept the new body

After I got the body on

And the most recent with the bed


Looking at the pics the front tires look like they are really far forward, which is not the case. they are about 1 1/2" forward of center but its really not noticeable when looking at it. The cab had to end up sitting a little different than I thought it would, which led to that difference. Just another reason to measure a hundred times before welding/cutting

 

do the front wheels rub any

 

No, like I said they are really not that close. I could fit bigger tires in there and still be fine.