There was an article on the "best fitting" sub-frame connectors for a 67-70 Mustang a long time ago. I can't remember now, who made them. They were a bolt on connector, that was welded anyway once installed. I welded a set on my brothers 69 Mach1 about 10 years ago. I did add a few more pieces of metal here and there, on the ends to beef it up a little. But basically, it had nice fitting and strong ends, that fit good, laid up against the floor-pan but didn't need cutting or go through anywhere, etc. It was 2x2" square tubing with 1/8 metal flanges that went around the front frame, and on the back, tied into the torque box and front leaf-spring mount... lined up with the bolt for the front spring, etc.
I want to say it was Competition Engineering, but I am just guessing. Maybe do a Google search... It is advertized as a bolt in, and could be done that way, but for the best strength, (the main reason for installing them) weld them in. Your rear quarter panels will thank you, if you do any hard launches with slicks...