I'll second what Mike B. said. Also, if you put in a BB, you may also need to beef up your cooling system.
My 59 1/2 ton Suburban has a 350 and SM465 (not quite as low of granny as the stock SM420). I have a 3.40 rear end in it (not as low as stock 3.90) and was able to get my 58 2-ton on a trailer moving along just fine. I weighed 17,340 when I rolled across the scales.
A 350 should have a lot more torque than the 283 (could also go to a 383, which is a bit longer stroke so more torque than the 350).
454 is doable, but may not need it.
