in the nuanced idea that the intake and motor go together and were probably purchased and raced that way until "upgraded" to the tunnel ram, sure.
the idea of several year old race motor in a year or two old car absolutely tracks with how things would play out back then. but another possibility, especially after seeing the motor broken down, is that it was purchased as parts, and put together with what all was available-- hence the alum rods, backyard balance job on the stock crank and the big ol gnarly cam. which again, under investigation of the trans, cut up floor, seats, and ladder bars kind of sort of tracks.
and as time rolled on, maybe your FIL collected the bits to make it more "SS" and less home brew?