Everybody knows that I'm no expert.
BUT, there are several 400 powered Stock Eliminator cars that are probably turning 8000 rpm or a little over, with D-port heads.
2 that come to mind are Randi Lyn Shipp's '67 Bird & John Schloe's '69 Bird.
And these heads/intakes are not supposed to to be ported. I assume they've have some sort of acid porting, which passes NHRA tech. So, if your rules allow full porting, correctly done D-ports should work for you.
The only guy running an iron head 428 drag motor, that I know of, is Carroll Warling. Norman Warling is a member here. He may be able to give you some info that would help.
Some of the quickest Stock & SS Pontiac engines are built by Parsons & Myers. Heads Up Cylinder Heads builds heads for lots of the quickest class racers in the country.
SS racer Greg Meyer has posted here that legal SS Pontiac D-port heads can cost up to nearly $10k. But, at today's prices, you can probably have a set built, for what you need, as cheap as or cheaper than buying a set of RA4 heads.
If it was me, I'd definitely talk to these Pontiac Stock & SS racers who are going quick with D-ports, BEFORE buying round ports. Just because one guy says it can't be done, doesn't necessarily make it so.
Bo Butner has probably spared no expense making Randi Lyn's Bird as quick as possible. I know the car has run low 10's. I'd try to find out who did the heads on her engine.
I'd also talk to members Greg Meyer & Norman Warling. Greg has run 9's with his 400 powered 3rd gen GT Bird. Carroll Warling has held the national record, with a D-port 400, & has recently built a 428 with #216 heads.
Scott Burton is another guy who has built some of the quickest D-port class cars. He also had the quickest ever RAIV powered Stocker. So, he's a guy who could really give you some info about making power with BOTH D-port & round port heads.
If you do go with round port heads, John Angeles may be the guy to talk to. His daughter Keri has run well into the 9's, with a '69 Bird. I think she 1st ran a 9, back in 2000. So, they've had 20 years of 9 sec experience, using round port heads.
Another round port option is the '68 RA2 heads. They should be a little cheaper than the RA4's. Have been several RA2 Bird Stockers run low 10's. 2 names that come to mind are Leo Glasbrenner & Bob Aceves. Leo has switched to late model cars. So, he may even have a set of RA2's he'd sell, or know where you could buy a set. And he probably knows all those Pontiac class racers out on the West coast. You can probably contact him thru his business, Remac Transmission.
https://www.remactransmissions.com/transmission-repair/