the bigest engine without major modifacations would be a fe series engine which the biggest being 428 in stock form same basic engine as 390 your best bet is to put a 351 windsor in it since parts are cheaper and more available for this motor
no problem if it there both just regular 3 liters or both the 3liters with overhead cams if there the not the same you will need the wiring harness and computer out of the one with the engine you using
detenation preignition (knockig sound rattling )under load or part throttle also buy running to lean you will end up with a mis fire 1or more cyclinders not working your engine will have to be rebuilt
check for clogged fuel filter there is a shredder valve on the fuel rail turn on key and push it in if gas shoots out you have fuel there probally fuel pump though drop tank unscrew big plastic nut remove pump put new one in and change fuel filter