HELP!! Straight 6 with headers, WHAT size dual glass-packs???

[1975stepside]

Hi, I've got a 75 Stepside with the 250 cubic inch 6 cylinder (with the integral head) and a set of headers I'm just ITCHING to put on it, but don't know how big the mufflers should be.

Any ideas? The headers split the exhaust into duals (3 cylinders per side), and I'd love to have a nice throaty rumble (not TOO quiet either!) . I figure probably just some glass-packs, "Cherry Bombs" or whatever I can find at the local parts store. But I don't know how long the mufflers should be to still have some decent sound to them.

Does anyone know? Any help would be greatly appreciated! Thanks!