Basin,
When I asked what kind of heads I was looking for Cathedral Intake Port vs. Rectangular (i.e. LS 1/2 vs. LS 3). From the ongoing discussion, I assume Cathedral port, the correct heads for a carb setup
This is a valid discussion for sure. The LS motors move more air (vs. old small/big block) for a given displacement. They can handle more carb vs. a similar displacement small block at similar RPM's. That said, there are plenty of LS calculators out there, but most will over carb for an airboat because they are aiming for max Hp at 6,300+ RPM (AKA zz4).
650 CFM is the small end of carb for a 408 LS turning 5,500. But the 540/540 numbers speak for themselves, that is a high torque figure. I would focus on tuning the 650 carb and your timing curve first as recommended. If it still doesn't give you what you want, try a 750 CFM, just expect less response in exchange for a bit more pitch in your prop at top end.
