hello im converting my 87 f-150 reg cab 8 ft bed into a flatbed can any one tell me how long the frame rails are frome the cab to the back so i can get two 4x4s to put on there for my bed to sit on. then next question if any one may know the answer wat would be the width of the rails an how tall the are with a 4x4 layin on them so ino how big of U bolts to get if any one can help id really apprciate it

 

You have some issues to deal with.
First the frame rails are not straight/flat from the cab to the bumper on an 88 F350 I would expect the 150 is the same.
You could spend more $$ on the bed than the whole truck is worth.
If you really want a flatbed, I recommend you go to a PULL-YOUR-PART self service junkyard and get a bed from them.
Fords are very common so flat beds are out there but most will be on at least a 250 so be prepared to cut and weld to get it right.
If you are really lucky you could get all the bits at the same time.

 

i have a flat bed built an ready to put on i jus have limited time the truck can be out of service. i gotta at least for now figure out how long the frame rails are from cab to back

 

an its jus a homemade wood one looks good i jus got to get two 4x4s to put on the frame rails so it wont sit to low