Internal balancing helps a little with crank & bearing life, especially with all-out race engines, but as noted, it's not really necessary for your application. It is very expensive. Weight is typically added to the crank for internal balancing unless the rods & pistons area lot lighter than OEM.

But you still have to get the whole thing balanced on a machine because of the mixture of parts.

I've been using Pioneer SFI balancers on my 360's for years. Mine are internally balanced so I have a neutral balancer/damper. But they have the counterweighted 360 version. Summit may sell them and Jeg's may have them under their label.

Pioneer SFI balancer #'s:

872038 for the external balance 360
872037 for the 318/340 and any internallyu balanced small blocks.