tire sizes?
 

how big tires can i put on my 77 f250 hiboy?

If you do have a Highboy you should be able to stuff 35's under it with no problems. The reason I say if is because a lot of people think they have one and don't. I've only seen a handful 77's but they are out there. lol. I had a 73 and a 76 and ran 35's and 36's on them though off road I would rub a little now and then with the 36's.

thanks. how can i tell if its a highboy?

Tires
 

I would start by looking at the transfer case if it is a divorced case or not meaning there is a short driveshaft between it and the tranny.

the transfer case is divorced. theres a small rod connecting it to the transmission

A small rod or a short driveshaft.

short driveshaft im pretty sure

Also I believe the fuel tank is in the cab behind the seat, which is a real easy one to see. I know over the years it could have been changed out. I know there was just thread on this subject, and the rear frame rails are different too.