Can I use B or C body manifolds fore better flow on my sleeper GTS?

Use b-body hi-po manifolds from a 70 or 71. The 68 and 69 drivers side will have issues with the steering shaft.
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I've also used C body driver's side and seen others do it too. Some have had to mill the manifold so the outlet clocks closer to the engine, but I did not have to do that. Since all these cars are a little different, your fitment will vary. You will also need to cut the steering column housing almost up to the firewall for the B body manifold and you will have to use floor shift for the transmission. IMO though, the B body manifold is the better one to use, even with cutting the column housing.