Definitely do the springs vs bags. Have swapped several sets out since the bags were a weak design anyway in the platform.
The trannys can be built and someone did a 4l80 swap, but for cost I would just build one. We are researching the swapping the TC. Think we got it worked out to put a mechanical lever in the center console. Hoping to find someone with PCM knowledge to allow it all to work together.
Some other issues to note is the rear control arms are weak, but tubular ones are available and I am quite happy with them. Also the panhard bar is not adjustable and the rear end will not be centered after adjusting height up or down. Adjustable one is available. Tie rod ends are weak, but straight 2500 hd ones are able to be fitted. Flipping or aftermarket front upper control arms may be needed to keep ball joint angle decent. There is one guy that makes the Stainless brake lines to replace the factory rubber. Not commonly avail due to fittings used. Factory gas tank skid is decent. The rest of factory skids are junk, but MDB Fab makes some steel or aluminum ones. Also the rear swaybar hangs really low on the rear diff. Several of us have pitched them completely and the stability is still pretty good.
Have helped build a couple dozen TB's into capable machines and would gladly lend a hand or info if anyone wanted to build one.