bolt in sub frame connectors

those additions have merit - but be sure of this - subframe connectors are proven to work on these cars - the ONLY discussion is whether or not to weld them in - and as it has been said, you should weld because once you've installed the bolt in ones, you will never remove them - the only caveat to that would be a later resto or redo -- but, that's what a cutoff wheel and a grinder are for...
Every person I have ever crossed that has installed sub frame rails has always said the most positive things.

I've always wondered about adding tubing parallel..i don't see*how it would help much but
as you say most feel that they do. ..I sure don't know.
I think torque boxes or trussing the corners would be a better modification.Nothing so strong
as a triangle!