Super CUSTOM EXHAUST?

I'm doing a custom exhaust on my 1980 Olds with a Chevy 350. I know back pressure has alot to do with horsepower, so this is what im doing..... I have new headers from Summit, along with 104 inchs of 2 and 1/2 inch pipe to run straight tru dual, 2- 28inch thrush gl packs and 2- 12 inch summit exhaust tips. On top of that I have a flamethrower kit that i'll be adding, i'll tack the sparkplugs in the pipe as I go. The exhaust in all is 144 inchs, but i took it down to 104 inchs because of the tips and packs. NOW the question: where should i place the gl packs at? Add any other helpful suggestions, don't be cheap let me know!