[Daves55Sedan]

The 6-cyl side motor mounts are different from the Y-block mounts, but you can get new motor mounts from some of the repro parts suppliers.
The radiator side mounting brackets on the 6 and 8 are different also, because the 6 is longer, so the radiator on the 6-cyl cars is thrust forward into that space you see at the top of the front air deflector. Also, the 6-cyl cars had a cutout in the front splash-pan for the radiator bottom tank.
You can keep all that stuff like it is and install a deeper fan-spacer between water pump pulley and fan so that the fan blade will be closer to the radiator.
Probably the biggest concern is the trans bell-housing and clutch. The 6-cyl bell-housing won't bolt to the y-block regardless of manual trans or Ford-o-matic.
You can use same starter motor and drive, but you will need to add the round rubber seal that goes at the back of the starter for the Y-block, also available from repro parts suppliers. As for the bell-housing, there may be a conversion plate available, otherwise you will need to get a Y-block bellhousing. If you have manual trans, you might want to look at your clutch size. In '56, Ford offered 9-1/2", 10" and 11" clutch sizes. I am pretty sure you have to get the y-block flywheel also, which will determine your clutch size (unless you wanna go drilling and tapping new holes for a larger clutch pressure plate and cover assembly). Same goes for the Ford-o-matic converter. The 6-cyl converter is smaller than the y-block converter and ring-gear.
Back in the old days, there were adaptor plates so that you didn't need to mess with swapping a bunch of transmission parts, but I don't know about now.