<VV> Opinion on rear coil springs

[Vairtec Corporation]

You are correct that the 1964 leaf springs are "weaker" because the 
transverse leaf spring carries a significant portion of the load.  You 
need to do one of the three options you outlined, because the current 
setup is dangerously wrong.  The 1963 coupe springs that you have on 
hand are not ideal, but they will be much, much better than what you 
have in the car now.

--Bob