Sorry for the long post, hope it's helpful.
I just finished installing USCT subframe connectors and torque boxes 5 days ago. Installation was done with the car completely disassembled and mounted on a rotisserie. I can weld but I'm not a welder if you know what I mean. Earlier this year we installed the same components in my sons 69. The 69 was on a lift, it still had glass, doors, trunk, hood but mechanicals and interior were completely removed. A freind of my sons who is a professional welder did the welding.
Welding the subframe connectors to the floor pan (thick to thin) is the most difficult part. The floor pan gets even thinner at the crowns of bends and beads. Avoiding welding on bends and beads will reduce burn through a lot. Welding on the rotisserie is much easier because the the car could be positioned with thick metal above the thin metal thereby acting as a heat sink drawing the heat from the thin stuff. In that case a continuous bead could be run by keeping the weld bead on the thick and quickly rolling it onto the thin then back to the thick. About 80% of the floor to frame connector is welded however IMO stitching were the floor pan is flat and accessable should be more than adequate. Sold weld all accessable areas where the frame connector contacts front and rear frame. On both mine and my sons a dutchman was required between the verticle portion of the frame connector and the transmission support
My advice is start with tacks and see if you can progress to small stiches. If you can't connecting tacks will work.
Practice positioning your head and mig gun a couple times (for each little area) before pulling the trigger, it's very easy to have a lot more stick out than you intended producing bird poop weld and burn through
Trim connectors to fit as close as possible before welding.
Take your time, if you get tired come back tomorrow.
The torque boxes are a lot easier. Continuous beads at frame rails, rockers, and frame to rocker supports and tacks or stiches where the front torque boxes contact the floor. There is a space between the front leaf spring mount and the back of the frame connectors. Plan for that, I chose to fill the are with another plate before installing the torque box. As CFD244 stated, some fitting required for front torque boxes including bending the torque box and bending the flange on the frame rail to rocker connector.