5.0 just not feelin right

ok my 5.0 in my mustang is a non ho and it has roller valvetrain and stock cam and not much anything else other than carb intake but it just dont sound right it isnt firing on #6 it is getting spark it has compression but i cant figure out why it isnt firing im thinking timing cuz it also revs slow and has an acasional back fire im thinking about getting a new EFI 5.0 HO out of a lincoln mark VII and switch it over to carb for my car i think that will be easier but i dont know but i know my current 5.0 is kinda pissing me off im only 18 and this is my first stang but ive loved them my hole life and this project is kinda needing to go quick cuz i ship out for the navy in feb. so any and all help is appreciated