I only worked on one 440 ci small block, It had 360-1's on it but I'm pretty sure it wasn't shifted at 7800. If you over rev at shift points It can slow it down pretty good.Converter is way off.Think of it this way the stall/flash should be within 1000 rpm of the shift point. So either you need a 6800-7000 converter, or you need to shift at 6000.

I would be curious what the cranking compression is.

A 440 cube small block with a 275 at .050 should make a lot of down low power.