Looking for F150 front cap and total bed/ two doors Truck is 1978 but everything I think from 1973 to 1978 will work

anything from 73 to 79 will work, only thing they changed was the grill and headlight/markerlight lay out 1/2 to 1 ton its all the same until you get into the suspension and running gear, check ebay, craigs list, local junkyards, hell im sure theres a farm or 2 with 3 or 4 fords that aint run in 20 years

They will all work, worst thing i have had problems with is usually the trim, my 75 had all the trim and some of the donor parts dont have all the holes in the panels to put it all back on. but if you have a drill it can be added or filled

I'm wanting to install an older model truck body on my 1978 F-150 4X4 I'm looking at 50's model anybody have one or know what problems I might run into?

bet you will be relocating body mounting brackets, best to have both complete trucks, and the ability to do a little fabrication, should be doable with a little welding. hopefully the overall lengths and widths are close so wheels fit in the wheel wells and you dont have to move the axle and get really carried away. some times they use the old frame and body and just replace motors and running gears so you get new brakes and stearing on it. good luck