The 42-48 Mercs have a long engine compartment due to the 4" longer wheel base. Should not be a problem to install a Y block or later 302 or 351 Windsor. Lots of area to move the motor forward if needed.
Using the stock transmission? The only interference is the oil filter, exhaust can be tricky. On a ford of that era the oil pan needs notched in the very front. The distributor will clear the firewall.
