Sounds like it's running out of timing to me.............

Do you know the procedure for the TBV timing adjustment ?
Not the same as a traditional distributor nor the TB IV. I have the procedure from a Merc bulletin if you need it.

fyi: Many people don't know that the Vortec heads take a longer reach plug like the Delco MR43LTS's vs the typical MR43T's.

Yes I do know the timing procedure. key off, jumper in set timing, key off, jumper out. Idle timing is at 9* with jumper and full in at 30* 3700 but my low dollar cheap ass advance light sucks big green donkey d*^%s. I have the correct, longer plugs in it. I have a new fuel pump, manual, installed. It made no difference. I have run it on a 5 gallon can to eliminate the siphon valve and boat plumbing. slight but insignificant difference. I have ordered an advance tape to double check timing.
I am tempted to just run it WOT and turn the distributor up until it pings slightly then back it off. That's the way we did it before advance lights. That should give a few of you a stroke.
I dont know. Its a 600 CFM carb, should be fine. Next step is to check which main jets are actually in it.
I suppose I'm asking if anyone knows if there is s rule of thumb, or standard answer that Yes or No to the need for re jetting with vortec heads over the old truck type all else being equal..
Thanks.

Camshaft and exhaust have more to do more with jetting than cyilnder heads do........unless you put a real good cam and exhaust in beforehand ..and then good heads :)

So I may still have a timing issue? is that the general consensus?

It could be fuel. When the fuel bowl runs dry from not enough supply (be it the pump or fittings or needle and seat or) the engine will usually lay over vs lean popping thru the intake like too small of jets or crap in the jets usually does.

As far as timing, you may need to read more into how the TBV works with mean best timing, rev limiter, and etc. Here is good info from Merc. Start reading .pdf page 16 on. ie; The wholeTBV section.
http://www.boatfix.com/merc/Servmanl/16/16B4R2.PDF

If your older motor died due to a valve seat falling out it was going lean. I had the same thing happen. With the Vortec heads flowing better and the same carb, and the motor falling on it's face at higher rpm you are lacking fuel. Jet up and see what happens. That is the safest thing to do at this point.

The high rise intake will also lean it out some. Personally, I'd swap to a Holley 650

Holley swap is a schlep. Edelbrock is the same thing. Ill go up jets and see what happens.

Research needle and seat sizing also. A few members have run into tuning issues due to the needle and seat not being large enough on the Edelbrocks/Webers.. Yeh, more hp than yours, but not a ton more.