Car vibration at freeway speeds; why?

if it was a slant car and was swapped to a v8 then theres no way it still has the original transmission, the bellhousings are way different. Accordingly the driveline may have been swapped. After 30+ years of use (or rusting in a field) sometimes the balance weights that are tack welded to some shafts jump ship. Also people forget to mark the yokes to the shaft when changing u-joints and sometimes that can be very critical, if it is, and its done wrong, u-joints will brinelle, thats when the needle bearings create a groove in the trunnion (the main cross) and the joints become stiff and usually fail fairly quick. Bent yoke ears can make the joint tighter than it should be. Ive had worn transmission mounts do something similar to that as well. Gunk in the inside of the wheel can throw balance off enough to notice at high speeds like dirt or the neighbors cat. A failing tire can distort at higher speeds, cords separating or chunking out, maybe a flat spot? Dont overlook something if it seems too stupid to be the problem, it usually is. haha!!