Any tips on fitting the US Car Tool Sub Frame connectors?

I agree with most of what's been said so far. I installed these on my '71 Dart, they definitely required some grinding and fitting. I probable test fit each one at least half a dozen times, grinding and fitting in between. I installed mine with the car up on jackstands at or near the suspension points, and I used a bottle and scissor jack to hold them in place while I fit them, checking the door gaps etc when I started welding. But that said, I have seen these connectors installed while the car was sideways on a rotisserie, upside down on a rotisserie, you name it. A bodies, E bodies, whatever. I have yet to hear of a single case where someone welded in a set of these USCT connectors on a rotisserie and then had issues fitting the body afterward. I'm not saying that it's impossible to weld the connectors in and warp the chassis, but I've seen dozens of installs via the internet that were done while the car was on a rotisserie, some being very high end restorations, and I've yet to see a car that had body fit issues because the connectors were installed on a rotisserie with the car flipped to allow for easy welding.

I will also say that it is a royal pain to weld the USCT connectors in while underneath the car. If I had a rotisserie, that's how I would install them. I'd lower and level the chassis out, use jacks to push the connectors against the floor and do all the fitting, check the door gaps and stitch in some welds at the ends and a few in the middle. Then I'd raise the chassis up on the rotisserie and flip it so I could finish the welding with the puddle below me instead of above me.