Newbie needs help

[CuStO67]

Hey guys im new to the forum. I am building my dad a 67 c10 for his birthday coming up in march and we have been working on it for 5 or 6 months now and now that we are working on the motor i am running into a problem. (of course) Ive been doing a ton of research on here and im sure that this question has been asked a million times but i want to get some opinions. I have a 67 swb that has a sb 327 out of a chevelle or a camaro and it has a 2 speed powerglide out of a 68 something or other. My question is that is horrible on gas and im wondering if just changing the gears in the rear end to something else like a 3.08 or somthing of that line will give it a little better street drive ability? I also have been told that i need a th350 or a 700r4? Just trying to save my self a little cash if i could just do some new gears or something easy like that.
Its funny because the truck started out as a scuff and shoot with just a few mods and now we have completely rebuilt the front will all new suspension parts, steering parts, and have completely swapped all to five lug disc. power everything lol.
Any insight into the tansmission would be great help as always this forum rocks.
Thanks