You have a lot of options, you could also go to a TH-350 which would give you 3 speeds intead of the 2 in the powerglide or with some more modifactions to mounts and driveshaft you could go to one of the overdrive automatics (th-700R4). Not an expert in automatics but being a chevy you have access to a lot of swapping combinations. You will probably also have to consider your rear end ratio as well.
