The 265 V8 is a great little engine. You did not specify but make sure your donor transmission is from a 265 V8 truck with Hydramatic. A 6 cylinder Hydramatic is a different bird.
Please check but I think the transmission gearing for car Hydramatics are different from a 1/2 ton trucks. Also bigger (1-1/2 and 2 ton) trucks had a lower first gear.
It also depends if you want a truck with all original parts (for show etc.) or a truck to drive around in for fun.
I don't know about the 57 but earlier 54/55 Hydramatic trucks have a different floorboard, starter, master cylinder, cross member and carburetor etc.
Personally, I would do the rear axle much later until you had a chance to make the truck in good running condition with the 3.90.
Your "Hydramatic rebuilder" should have the tools (throttle arm setting gauge, band adjusting tool, hydraulic pressure gauge and how-to-manual) to set it up.
You need to experience what a normal running/shifting truck is like (establish a baseline) before you change the rear axle.
Then it's like Jerry said, it will need tinkering.
