If you post the VIN number here, there may be someone on HotRodders that can help. There may also be a few F100 specific sites out on the internet that will offer more support, perhaps, than this site will. I know that in '56 the only V8 pick-up trucks were all y-blocks. This is the 234(?), 272, 292, 312 series of engines. There were also 6 cylinder trucks. I do not know if the sixes and V8s were readily interchangable, or if engine mount welding was necessary. I do know the first FE motors, the 332 and 352, were introduced in 1958. And I think that the pickups offered both the y-block and the FE for several years. I had high-school friends who put FE motors into '56 sedans, and they worked great. They bolted right in where the y-block used to go.