You're 100% right about the big block cars having 8 3/4s and all 340 cars have them as well.Other small block 4 speeds are hit and miss,I've seen 273 and 318 4 speeds with either a 7 1/4 or an 8 3/4.
I don't think the heavy duty suspension option would give you an 8 3/4 but ordering the trailer...