No, The v6 tranny won't hold up. You can get a used 200r4 for a couple of hundred dollars.
If you have a 200r4 or 700r4 you're safe.
I had a 86 Firebird with a 2.8 v6 and 5 speed, and swapped in a 350 with a 3 speed auto.
I lost 1 mpg and was a lot faster. If I had went with the overdrive tranny, my mpg would have increased.