I had a response the other day for you, but we lost internet and subsequently lost my response. I will try to replicate it for you here.
I think your current heads would be pretty decent for forced induction compression wise, but as I'm sure you know, they don't flow very well at all compared to today's offerings. Personally, I would invest in some heads before the supercharger. There are a plethora of options out there from Edelbrock, Trick Flow, and RHS to name a few for every budget and power objective. It sounds like you have some money tied up in the short block, the heads are going to be a choke point no matter how you approach it without a lot port and valve work.
Here is an article from fordmuscle.com testing a number of SBF heads. They list the specs for them all as well as give flow numbers. While the D8s aren't listed, I'm pretty sure they don't flow quite as well as some of the other stock offerings in this comparison. They also test a set of "home ported" heads. They just won't measure up to a good aftermarket head.
http://www.fordmuscle.com/archives/2000/05/heads/index.php
Also, as I'm sure you know, the engine is basically a pump. The more air you pump in and out, the more power it will make. A supercharger magnifies the effects of everything else about the engine, so if it is built to flow it will make power. As we already determined, the heads you have really aren't good for that at all and you will need to do some work to make them "acceptable." So, if you can swing it financially, a set of heads aren't a bad investment whether you intend to supercharge your engine or not. Then, if they are a choke point, you can always do the work that you inherently need to do already in the form of porting/polishing.
With heads now-a-days claiming 50+ bolt on horsepower (again, not an apple to apple comparison...), imagine your car with instead of 210 to the wheels, it had 260. Then, add the supercharger. Estimates may range from 25-50% increased power output depending on the application. Let's take a conservative number, say 30%, and add it to these numbers. 210 + 63 = 273 hp, or 260 + 78 = 338. That's a 65 hp difference overall just because of a theoretical head choice.
And I know our cars are light, mine weighed in at about 2800 with my fat ass in it on a scale iirc. And we know that power to weight ratio is where it's at. It would be a good time to at least debate buying aluminum heads for the engine. They will both increase power and reduce weight, which in theory, will make the car quicker/faster. Another good point for a better set of heads.
Now I realize I'm here on my soapbox and not in your shoes either financially or where you are in your life. But it would be a good investment all around to look into a set of heads. If money is the problem, Speedway has a set of Flo-Tek heads that aren't too shabby as far as magazine testing goes. They aren't the best, but definitely better than the anchors you have now. And they aren't too expensive, <$800 for the pair. You will more than likely, of course, need to buy different rockers.
https://www.speedwaymotors.com/Flo-Tek-203505-Small-Block-Ford-Aluminum-Cylinder-Head,46332.html
And here is a link to a magazine article from Hot Rod that used them in a comparison with some of the bigger dogs in the "economy" aftermarket world...
http://www.hotrod.com/articles/ccrp-1302-six-budget-ford-heads-that-work/
Now, if you were to add this to a rear gear swap and a possible converter upgrade, you'd have a potent little machine. I'll get off of my soapbox now......
P.S. Speedway is having a promotion starting today, spend $200 and get a $20 gift card. They also give $10 off for signing up for their email list. So if you were to get them this week, they would be closer to $765. Not a bad price at all.....
(I wonder if you bought them on 2 different transactions if you would get 2 gift cards??
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