[dwcsr]

Lets go over the numbers and it will become apparent why 200r or 700r vs th350.
I am only talking about 1st and 2nd not direct or overdrive and does not take into consideration the rear ratio

Trans ratios

Powerglide 1st - 1.76 2nd - 1.52 (Direct)
TH350 1st - 2.52 2nd - 1.52
th400 1st - 2.48 2nd - 1.48
200R 1st - 2.74 2nd - 1.57
700r 1st - 3.06 2nd - 1.62

Stock 3 speed standard truck SM318
1st - 2.94 2nd - 1.68

As you can see the 200 and 700 more closely replicates the stock low gear second gear ratios. If you change the rear gears this changes your choices for transmission
200 and 700 of course have an overdrive and all transmissions had 3rd as direct 1:1, PG is the exception

Trying to pull out in traffic with a 90HP engine and a tall 1st like 2.52:1 could be a problem so some.