GM cars of that period had extremely strong frames. They were of a "top hat" style and as long as its not twisted or rusted through it will serve you just fine. I ran my 41 Chevy pro-street car (454) on a stock frame with a Mustang II ifs and disks. Believe me, a V6 will not overpower your stock frame.
"The best car safety device is a rear-view mirror with a cop in it." - Dudley Moore