could always rebuild your TH350 also an replace its week links.
in my opinion the 700r4's are junk plus need wiring to get the overdrive. but ods are 50/50 on finding a good tranny from a junkyard plus 700r4 valve body valves like to stick. motor revs sky high before it shifts. i would say a TH350 is stronger than a Th700R4 plus your truck is set up for a TH350 so why not stick with it but improve it to handle hard abuse with B&M parts