I think you can have a LOT of fun with a turbo'd slant 6 and ten pounds of boost!
That will gIve you the capability to run a stock head, pistons and rods, cam and practically everything else except for the intake system and turbo-mount on the exhaust manifold.
I'd think that 300 HP would be achievable with that stuff. That will give you a car that should run toe-to-toe with a 340 V8-powered A body car such s Dart or Duster as long as it's stock. AND, FOR LESS MONEY...
I am told that vacuum secondary carbs will not work in a blow-thru environment. I odn't KNOW that it's true but I have never read otherwise.
My 2 cars that are blow-thru are both equipped with 4bbl Holleys that have mechanical secondaries.
Having said that, I'd also like to say that for a buildup like yours, I believe that the better setup would be a Holley 2bbl, like a 350 or 500. That will be plenty of carburation for 300 HP and has much less complication than a 4bbl. The intake manifold would be cheaaper, too, with a Super Six manifold filling the bill, nicely. They's cheap, too, compared to a 4bbl aftermarket manifold. The carb;s cheaper, too...
I WOULD recommend an alcohol/water injector from a reputable manufacturer like Snowperformance, as it should allow you to run premium pump gas without detonating the ring lands off your pistons, and blowing a head gasket. Those units are $300.00, but it's money well-spent on a car like this. They really work...
You won't need to change your rear axle RATIO, for best performance as turbo's slant 6's seem to like gears in the high 2's... so, if you have a 7.25" rear, you might want to think about just going to a junkyard and pulling a complete 8.25" unit (plentiful and cheap) out of a late Duster ('72, 3, or 4...) and bolting it right in. Pleant strong!!!
Wait; I don't know what yuor car is, but I'm assuming it's an A body, right? LOL!
Welcome to te "club!" Boost is addictive!!!:cheers:
BILL