In my 1977 f150 i have a 351c and was wondering if a 351w out of a 1996 f250 would matchup with the engine mounts and fit the transmission?

 

Welcome to FTE. There is a lot of details that you need to give in order to get correct information. Are you 100% positive it is a 351 Cleveland (351C) and not a 351 Modified (351M)? Is it a 2wd or 4wd? What was the original engine since the 351C was never a factory option in '77. So I am going to give you two scenarios-
#1- Your truck is a 2wd and it had a 302 (V8) or 300 (I6) with a C4 transmission in it from the factory. Some previous owner (PO) decided to install a, no crap, 351C which would fit since the 351 C has a small bell housing bolt pattern. In which a 351 Windsor (351W, 5.8L) will most likely bolt right up to the tranny and motor mounts. Seeing as how the '96 has EFI, you will have to find a carb and intake to match, also I believe that the distributor will need to be an old model as well, think Duraspark.
#2- Your truck is a 4wd or 2wd that came from the factory with a 351M and a C6 transmission. In this case, what you most likely have is a 351M and not exactly a 351C. The 351M is what actually came in these trucks, not the 351C. All 351M came with the bigger transmission bell housing bolt pattern that a 351W will not bolt to.
Now, if you have the complete '96 truck, you can spend the time to swap everything over. Including the tranny, EFI, and everything else. This would be a worthy build to grace this fine website with and we would all love to see. Another thing that helps tremendously with any problem is pictures. The more the merrier. I hope this helps you.