I've got a chevy 355(325 horsepower) in my 620 now(had a 327 when I bought it 5 years ago)..... My firewall was cut, and a 6" "dog house" was welded in. But I need to shorten a Ford 9" rear first, as the stock Datsun rear cannot take any abuse(blown up 3 so far).
I just purchased a GM L33 aluminum 5.3 Litre with a Corvette LS6 top end....will probably start on that swap next year.
A typical Gen 1 small block chevy can be done, there are a few around here... as far as the Ford Coyote/Modular 4.6/5.4 litre....I think it might be just too wide(at the valve covers) to fit in a typically stock Datsun engine bay. A small block Ford has been done too, with lots of room to spare(disty is on the front).....up to you, and your capabilities/budget!!!