the block would yes.
depending on the casting no.s williams are F7R 700
the meganes are F7R 710, and F7R 714, both the meg blocks will fit straight in, the head will have to be removed and swapped from the williams....unless you want a load of hassle trying to mate the williams/meg tps , lack of dizzy etc. the 714 running an internal drive for the oil pump, taking it off the crank rather than belt.
id say 710 is the better one to go for tbh out of the 2 meg lumps. both my williams' run 710 meg lumps now.