Will a Y-Block fit the '49 to '51 Ford? In a word YES! In the '60s I remember my neighbor had a '49 coupe with a 272 in it.Today, one of my club members has a '51 tudor with a 312 in it that he also built in the '60s. It's a semi custom that he used to race at the track and it ran mid 13's with a single 4 bbl. He now runs a tri power on it. He's not on a computer so I'll have to tell you what he told me.
He took the motor mounts & frame brackets off of a '56 ford car, used the truck oil pan & pick up tube, used the 56 radiator and brake & clutch pedal setup, (it's a T-10 4 speed with a thru the firewall brake master cylinder). Bolt the motor mounts and frame mounts to the engine, slide it and the trans in the engine bay and position the engine/trans to line up with the transmission mount. Weld the mounts to the crossmember.
When I get more info and photos I'll post it on the Fabulous 50's Ford Club website accessible from this site's links page.
To answer Gerry's question the Canadian '54 Ford still used the '53 frame. Many of the Canadian cars also came with the 255 cu. in. Merc flathead instead of the 239.