Find out if it was originally a slant or a V-8 car. The fender tag will tell, as would looking at the transmission bell-housing. You can convert a 65 from a slant to a V-8 easier than 67+ cars, if that is your desire. Many posts on that here. Some have installed a Magnum engine, which gives you MPFI and roller cam fairly cheap. The matching tranny is harder (must cut & patch tranny tunnel on body). For dual exhaust, you must cut & weld the tranny cross-member. A V-8 swap usually requires a tougher rear-end.