Lee, changing to a flexplate and bolting up the TH350 is easy. When I originally put my truck back on the road, I flipped it to a 350 from a three on the tree (it was seized).
The starter is the only thing I could see being a problem. If it's already block mounted, then no problem, but if it's trans mounted, you better check that your block is drilled for a block mount starter, since the TH350 uses block mount. I don't think you'll have an issue with a 1970 250.
You'll also need to add a tranny cooler (your rad may not have any built-in cooler), plus you'll need to hook up the kickdown cable. Make sure the 350's vacuum line is hooked up to good manifold vacuum. I originally teed mine into the PCV valve hose (it was convient at the time), but this didn't work because the PCV valve bled off all the vacuum.
Good luck,
Greg
