I have a 79' Scarab 300 with twin 330hp 454 TRS drives at WOT they are running at 4000 rpm with 21 pitch

Sounds like something isn't healthy with your Scarab. You should be running 46-4800 and about 60 mph. 21's are the right props but something else is wrong.

It still has the old log style manifolds and exits through the drives. I'm getting ready to put center rise manifolds on and run it through the back. I'm hoping that will open it up some.

Did you just get the boat or is this a new problem? What is your speed on GPS? How is it runing, stumbling, rough, smooth or just down on power?

I got the boat at the end of last season. Only had the boat about a month before winterized it. Mem Day weekend 1st time out this season. No GPS yet, felt smooth, 1st time I opened it up, water was pretty rough here at LOTO could not trim up the drives port gauge wasn't working, found the wire on the sender was cut, need to put mech gauges on. Had a guy ask me over the winter what RPM at WOT it was running. This boat is a work in progress needs alot of updating need to get it out of the 70's lol.