Tie-bar lifters exist for the conversions. They’re not cheap, but the lower end of those run the risk of being trash in today’s economy. Haven’t heard anything bad about Howard’s yet.
Best bet would be to get a 86’-up 350, ditch the heads if they’re swirl ports, have a shop magnaflux them if they’re Vortecs. Keep an eye around swap meets, but don’t count on getting lucky there. 5.3’s are LS based, the Vortec SBCs were only 305s and 350s.
200’s have a lower numerical gear ratio for 1st which lends itself better for drag strip use, but both the 700 and 200 require some upgrades in hard parts to hold torque. 200’s are more of a fine art, and 700’s tend to be a minefield in the aftermarket.
Couple tips: clutches and a shift kit don’t make a transmission capable of holding 700hp, and stay far away from Monster Transmission. If you go 200, stick to the known turbo Buick builders.
4L60E is a waste, requires wiring. The 4L60 (no E) is just a 700R4 with a different designation.
If I say this once, I’ll say it a thousand times: for the price of a G-Body 8.5, find a Fox 8.8, rebuild it, get a swap kit, get 5 lug axles, stick to drums and be done.