If you want torque like you are talking about, you might want to consider a Pontiac 400. They are super torquey down low because they have a long connecting rod length. It is possible that you already have a dual pattern bellhousing, but if not there are adapters for Chevy pattern to Pontiac bellhousing bolt patterns.
I built a Chevy 400 for my '73 Camaro. It has extra torque that the 350 does not have (possibly another option for you). I was very close to buying a Pontiac 400 before I got my current engine. I mean really close. Out of all the engines I have played around with, they are the torquiest down low and the have the quickest 0 to 60 times. Of course all things being equal.
Another good thing about the Pontiac 400 is that you won't need to change much to make it how your are wanting it to be. I have seen them make cars slower by adding larger cams. If you build it with around 10:1 compression, and keep it basically stock it should get good mileage with more torque than you will get out of a 350 Chevy at the lower RPMs.