Well, let's see--- your '68 is a post car (therefore stiffer initially) has a shorter WB, less rear overhang ( I think) and you Could run anything from a 6cyl to a BBC.
The '66 is overall smaller, percentage-wise more overhang, less frontal area (although squarer) & can accomodate any engine also.
I weigh each one to be sure, measure the frontal area also, THEN make the decision. This sounds like a racecar project & function usually takes prefernce over form.......