[PeteH]

I have a similar driveline in my ff 289. 65 289 hipo (6bolt bellhousing)bored .040, flat top pistons, original iron heads that were highly modified in 67 for drag racing with a nice port and polish, and SBC valves, BBC valve springs. I converted to roller lifters and Trickflow roller tip rockers, using a fairly aggressive Comp cam. Edelbrock FB4 intake, Holley 670 carb. It screams, nice cammy idle.
I also have a 64 BW T10 with 25 spline output. Not the best for clutch dumps or hard shifting as they were known to break that small diameter output shaft. Have a 28 spline toploader that will be rebuilt and swapped out maybe next winter. Am running a 60s era Lakewood bellhousing with external hydraulic slave actuation. 8.8 Ford IRS with 3:08s for the time being. Have another pumpkin that will be built with a posi traction diff, and slightly higher number gear ratio.
I would suggest you consider a toploader for the robustness of design.