That's right, both 307 and 308 HT GTS had a single exhaust standard, but he from memory wanted the 307 as 461's and 4BBL manifolds and Holleys and SBC cams were cheap in 1969/70, plus dual systems were available either from GMH or aftermarket. There was bugger all aftermarket stuff for the 308 and not much worked with them anyway other than the exhaust and a cam, manifold and Quaddy were pretty good. The 307 with 461's, dual system and a 4BBL Holley would leave a HT 308 in its dust given the lack of aftermarket stuff for the 308 then. Both would have suffered from the standard Powerglide though. The 461 Fuellies would bump the 307 to around 9.5:1-10:1 compression.

Remember the Aussie $ was pretty good back then and it was easy and cheap to get 2nd hand bits in. There was a booming aftermarket for SBC bits, and the 308 was in its infancy. John Cain used to bring pallet loads of fuellie heads into Australia in the 70's, and they were bought for a song compared to today. In the late 60's the early fuellies were well over 5 years old already.