Looking for some Differential advice

Can you shift it to second gear? Do you have kickdown linkage hooked up? I daily drove a 1bbl slant down there for years with no issue.

If the slant is hurt, you might want to wait on the rear end swap and focus on the motor.

Like rusty said, you might be over carbed, tall gears and tall tires. Worst possible combo. We need more info on the setup though.