Looking to buy/build 500-600 fwhp 360

I have a question about going from my current externally balanced 360 to an internally balanced 408. Will I have to do anything special to the existing flywheel (it's a 4 speed) when I swap in the new motor?

The car is having the differential narrowed. I am told it has truck axles. It looks like 295's will fit under the rear. Differential and glass are going in shortly.