swap with the truck engine would be best but im not sure why you would spend the money to do this is like what 20 more hp and a little more tq.
why not spend the money on upgrades for the 3.8?
get a locking diff and if manual get 3.73 gears that will wake that car right up!
a tuner can get you that extra power along with and exhaust and intake.
just weight your options if you want a little extra power look for upgrades for the original motor because after the cost of a 4.2 truck engine and the time to do it it wont be worth it when you could do a few bolt ons to get you there...
i would say yes to a 4.2 if you were going to put forged internals in it and boost it but just wanting a little extra power just look around for bolt ons
1973 Mustang coup:
New ginger interior
Rebuilt 302 and c4
2.5in exaust w/flowmaster 40's
2012 chrysler 200s (DD)