Brian,
Here's a thread on the E-Tec Owners forum you might want to check out.
http://www.etecownersgroup.com/post/...03692?trail=15 This owner had a 300 on a big 30' aluminum boat, and he's a mechanical engineer, so you might join the forum and send him a PM. I'm sure he know the weight of that rig and how much load he was carrying. You might want to pose the same questions to that Forum also, as it's excellent, similar to CSC in the wealth of knowledge there and friendly folks willing to help any way they can.
Skip and DonV said the Sea Mistress had PLENTY of power with the twin 250 HPDI's on a bracket, even though Carl said they were mounted way too close together, as most twins are on the SeaCraft hulls. He always mounted the twins in the center of the center panels on his race boats to get them in the cleanest water. You won't be nearly as fast with a single 300, but you'll only be feeding about half as many cubic inches as that 454, so I'd think fuel economy would be fairly decent, especially when you lose that much weight!
What sort of bracket did you get? You might want to consider a jack plate, because I don't think you'll have to worry about stern weight, but getting the engine height correct will make a big difference in performance. The AV plate wants to be out of the water. I'm still at least an inch too low even after Don redrilled the motor mounting holes to raise them an inch, with bolts in the bottom holes on motor bracket. Denny