Sounds like a great project. I put a manual valve body th400 trans and a Gear Vendors overdrive in my '86 Vette. The Gear Vendors is great. It drops my rpm's about 500 when the th400 is in 3rd (1 to 1). I've got a 3.07 rear diff. The guys at Gear Vendors were great to deal with and very knowledgeable. For sure give them a call and ask to talk to a tech and explain your setup and see what they say. I'm glad you've joined the forum. This build has lots to look forward to.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio