you would be correct, the 200r4 and 700r4 are both longer, a crossmember and extension kit are needed i believe, probably easier to change gearing to get a better holeshotSwapping gears would help more than swapping trannys. If you drive on the highway a lot do a 2004r/3.23-3.73 swap. Gets a good hole shot and good rpm's on the highway. Don't forget about drive shaft length. Let the guru's chip in but I think its a different length from a Th350 to 2004r not sure about the 700r4.
I was thinking along the same lines, the OD in the 700r4 will turn a 2.56:1 gear ratio into something like a 1.7:1 in 4th gear and that is gonna mean its a gear that can only be used going downhill (and depending on the motor) on level roads, but very few motors short of big blocks will have the torque to pull that kind of gear ratio up even the slightest hill.earlier this year, i swapped my fourth gen over to 700r4,)2.73 gears, first and second were amazing, but fuel mileage went way down, 60 mph was around 1100 rpm, (so lugging in fourth at 55 gets old) swapped to 3.73 gears and have never been happier, really woke it up! now with a tbi 350 i get 22-25 mpg around town and way better on the highway.
2.56 gears might not be so bad, but thats still pretty high for an overdrive.
the only things i had to change was driveshaft cut 3 inches, moved crossmember back and drilled new holes(90s blazer trans mount) and on mine had to change kickdown bracket(although it would have been as easy to change kickdown cable instead but i already had the other bracket.)