I'm with Gerry; bought some really ****ty 990s from a member here ( all the gear was toast), paid too much, and they sucked poorly down low, AND up high. But I left the boat anchors on there until I got a set of homemade 781s done up. WORLDS of difference. HOWEVER, it was still a 427, just down on TQ and HP across the board. If it wasnt a tripower atop, I likely would have fattened up the jets to counter the weaker signals to optimize it , and let it be. But I changed the "fluid dynamics" instead, by putting heads on that will maintain better charge velocity.
Oddly, and to the OP's point, had to run a rectangular port tripower atop oval port oem 781s. Now, from a fluid dynamics POV, when you constrict a passage, the flow's speed and psi rise at that point - which entering the cylinder heads isnt a bad thing, and maybe why this old "racers trick" works oddly well. The flow hits a friggin' wall, but then sucks through a smaller hole. Velocity + . It goofed me that it worked, even at low speeds.
Your flow doesn't hit a wall, but simply pukes out into a bigger pipe, a pipe that is frankly too big to begin with. The worst possible scenario before it hits the valve bowls. Velocity - + another - for the too big intake ports. so velocity --
Get the right intake on; it ought be a much better matched system. Recall that all the old timers call these nothing but "air pumps". Your's may be losing much intake velocity and flow, due to a port mismatch.
$300 to swap it out. Easy job. Hardest thing is to wait for the coolant to drain.
you may net up to 30 ponies tho, and more importantly, it'll make "seats of the pants' lite a small fire under your azz!
And also with Gerry on cam timing. Do the quick and dirty method, only need to remove valve covers and ignition + to roll it over w/o firing. You are looking to ensure the #1 intake valve is open more than the #1 exhaust valve at #6 TDC, if that makes any sense. Its EASY to check. Measure the relative heights of the #1 valves when the engine is in #6 TDC ( one revolution out on the balancer). If intake is open more than exhust is ( literally valve retainer to head surface measurement - crude) , its advanced. Pretty much where you want it. If they are even heights. the cam is "straight up". ANd if the intake hasnt cleared the ex height yet, its retarded.
PS: Another Pro story, I told a pro engine shop that was freshening up my 427 to put my Ultradyne in +4 minimum. Well, i couldnt get the thing to idle at all hardly,. 1200 rpms, 7" Hg, etc, so I called the cam designer Harold Brookshire, and he 'splained the QnD method. Well LO and Behold, wasnt the cam in -7 degrees, as the exhaust was still more open than the intake valve. ( which I found after the QnD said it was retarded by using a degree wheel. ). The pros had put the offset cam pin bushing in backwards. Literally. WTF? Pros, eh? I did the engine myself in my garage next time, with machine work farmed out and checked with my crude dials, calipers and plastiguage.
Anyway Harold's QnD saved me a ton of anguish.