Need some tranny advise for 76 GMC.

[Streetmaster]

I've got a 76 swb, twd, 1/2 ton pu, that had an inline-6, with a saginaw 3 speed, and three on the tree. The motor was spent, so i decided to replace it with a .030, 9.0:1, 350sbc, with a performance retro-roller comp cam. The saginaw 3 speed, flywheel, and clutch assembly fit right up to the 350 sbc. This transplant was done last summer, meanwhile the trucks been sitting all winter long, and im getting ready to start working on it again. But thru the winter cold crap, i had thoughts of going with a TH350 instead of the saginaw 3 speed, and would like some oppinions on the pro's, and con's, of either the stickshift, or the automatic, in a 4,000 lbs. pu with 3.42 gears. The saginaw 3 speed, has a 3.11 first gear ratio, and the TH350 has a 2.52 first gear ratio. The truck will be a daily driver, with P275R60 tires. Please Advise ThanX