He's got a 3600 and they do not make 3:55's for them. Some guys are using a 1967/72 Eaton HO52 pumpkin from a C20 (3/4 ton) or HO 72 from a (1 ton C30) These come in 4:10 gears which should help lower the RPMS and give you a better freeway speed. Do some research on the bolt, lot's of information on doing this. You might want to ask this question in the driveline section.

Here's a Great write up from our sister group