Re: considering 1991 F250
my 84 had a 4.9 in it. swapped it out for a 351 and wished i never did. had alot of problems with the 351 granted the 351 had high miles so i cant kick it but the 4.9's are tanks. just check over for leaks and what not thats about all you can do besides listen for noises. i mean you can kill the 4.9 but you have to work to do it, great motor.