From straight 6 to a 4.3 V-6 costs

[76GMC1500]

Once you go Volvo, you'll never look back. *They're way nicer to drive (cone clutches), way easier to maintain (the only time the outdrive needs to come apart is for the bellows and the cone clutches eliminate a lot of shift cable problems), they seem to be lower maintenance, but they cost a fortune to fix when they break (which is rare). *Of course, these are the old Aquamatics, I haven't driven the current SX drives.

It would probably be best for you to keep the I6 in there now, though. The inline configuration really makes the engine easier to work on. Otherwise, if you're going to go 4.3, you might as well go 5.0/5.7L. You obviously have the space since you'll be going from a 6 cylinder long engine to a 4 cylinder long. Except for the length, the dimensions of the 4.3L and 5.0/5.7 engines are the same. Parts are mostly interchangeable between engines but you'll find some parts for the 5.7 cheaper.