Welding cage to rear sub (camaro,nova, etc)

After a couple year break, I want to finally build my leaf 75 firebird. Not sure exactly which build yet, but a couple questions to help point me in the right direction.

If I chose a track that requires a stock rear subframe, what is the easiest way to either separate the body from the rear sub, or weld my rear kickers down, cell mounting etc?

Im allowed an aftermarket body, so just not sure how to do so and keep the stock rear sub.

Thanks!