I agree the 4500 plenum would be much better. You could grind on the pro comp intakes but I think what you will find is they are very porous and have a lot of sand in them. At least that's what I found 10 years ago when I was setting one up for 16 injectors. Talk about a welding nightmare.

I don't know of any. The way the bolt pattern is they would intersect each other. You can get a 4150 base plate that will fit both.

My experience with dynoing the old engines and the new ones with multiple combinations of carbs, intakes and spacers leads me to believe you're going to see the most improvement by using both the bigger intake and carb as a set. I'd even take it a step further and suggest a spacer as well depending on the intake. I believe if you take a 4500 manifold and mill it down and then put a 4150 on top of it you won't see enough improvement to make it worth the effort.

My engines (both builds) have smaller heads (305) and cams, so I'd assume yours should be capable of more flow. My 4150s were from Pro Systems and actually flowed 986 cfm so they were a decent size. The 4500s are Quick Fuel marine and were rated at 1050, but I don't know the real word flow. Here's the highlights of the induction changes.

Cam averages 239/641
Vic Jr 4150 / 4150 no spacer
644 hp 654 tq

Vic Jr 4150 / 4150 2 in. spacer
660 663

New cam, otherwise same parts. Cam averages 243/654

Vic Jr 4150 / 4150 2 in. spacer
674 645

Vic Jr/ 4150 with 2 in. adapter and 4500
686 659

Vic with 4500 plenum (same height as the 4150 version) no spacer 4500 carb
695 664

Same 4500 intake and 4500 carb 1in spacer
699 673

Same with 2 in. spacer
706. 666

I ended up going with the 4500 and 1 in spacer. Gave up small amount of peak for overall better hp curve and more torque.

I missed or forgot your cam change...can you add that into above post ? And the exact carbs used also....thanks !

I opted to change the cams when I rebuilt the engines as the original cams were spec'd for a boat with totally different exhaust and drive limitations. The new cams have about the same duration as Tim's with slightly less lift. The old cams were smaller yet. Both cams are from Bob.
Edit- the AVERAGE figures for the old cams is 239/641
The AVERAGE for the new cams is 243 / 654

Were those 4 hole or open spacers ?

I tested 4 hole and open on the 4150 combo and it made no difference so I used open. I only had 4 hole HVH to test with on the dominators so that's what I used. Never tried an open on those.

I plan to flow intakes and heads together and see whats gonna work best, doing some extra homework this time, and a lot more little tweaking, not just bolting them on like before, the heads and intakes will be massaged as needed, my buddy will decide what intakes to start with and go from there, I am doing this one last time, not gonna swap parts all around or try different cams and such, flow heads, grind cams, done... dyno if cash and time allows, if not the gps will tell me.

my buddy already told me a 1" spare of some sort will give me best results overall, need to find the space as I have hatch clearance issues...

Excellent info for sure, good to have a log og it, if I dyno I may play spacer and non but intakes won't be changed as whatever I build will be matched together.