horsepower guesstimate?

No.......... First of all, this is the Talk forum and you need to go to Tech. Second, the Performer intake is pretty much just a stock replacement and you need to step up to a RPM Air Gap at least. Next, your heads would be holding you back from making much over 250 h.p., and last but not least, you need to go to a dyno to get your timing and fuel dialed in. That alone could gain 100h.p.

he's right about the intake for sure, you could get a ford racing intake, the one that is a copy of the old shelby intakes (can't remember the number) as that is about the best dual plane you can get for a small ford, or the rpm air gap is a good choice too. the heads are definitely going to hold you back some as well, but they aren't horrible, they aren't great either though. i'd say you're looking at about 275-300 at the flywheel, which will be about 240-250 at the rear wheels.

bnickel's right. The intake is/was known as the F4B and is a copy of the old Shelby dual-plane, very effective part. I wouldn't get too jacked up over the 271hp hi-po heads. All early small block heads are identical, with the exception of the screw-in studs that the hi-po's had and they are all pretty lame compared to off the rack heads of today. Ported heads are only as good as the guy porting them, so you may want to consider that. Personally, I'd venture a guess of about 250 or so horses, give or take. I seriously doubt ANY small block Ford ever got 271 stock, regardless of the factory ratings or any urban legends. I remember when Hot Rod magazine did a shootout between a 271 horse '65 fastback, a '70 Boss 302 (290hp) and a 225 horse '89 GT 5.0, it wasn't even close between the vintage cars and the '89. But then wouldn't you expect Ford to have learned a thing or two since '65?

I don't have experience with the xcelerator, but I know that they are awesome intakes. If they had it for a 351 Windsor i would get it. You simply can't beat single plane intakes. I have heard that they are the top intakes for 289-302's.