The largest motor that 'bolts' into the 66 is the 351 motor..in both the c and the w flavors. Both of these can be stroked to 427 or so cubes for a huge displacement race setup.....393-408 is more streetable and longer lasting.
You can go to a big block by converting to a Mustang II suspension and removing the shock towers...after that...you can pretty much put anything you want in
Probably a 454 stroker on a aluminum block or I think darts block can do it also.
More cost effective is a 434 I think it is from major stroker kits on 351W blocks. Realisticly a 408 is what Id do though, otherwise your rod ratio starts getting kinda high unless you go with what I spoke of first, a race block.