300 HP isn't an unreasonable goal. If I was looking to get 300 HP out of a carbed 302, here's what I'd do:
Edelbrock Performer RPM intake
GT40 aluminum cylinder heads
Crane 2031 cam
1 5/8" shorty headers
Hi-flow catback exhaust (Flowmaster, Dynomax, whatever you prefer)
Edelbrock/Holley carb (650 cfm)
In addition to those mods, I'd have the GT40s milled .060 to raise compression to 10:1.
However, to achieve the clearances needed, you'd probably have to replace your pistons with soemthing aftermarket.........and being that you can choose your compression ratio, milling the heads would be unnecessary. They'll probably need changed anyhow, if you're planning on using the GT40 heads. Most of the pre-1986 Ford 302s have big time clearance issues, even when attempting to use a stock 1987 E7TE head. Reason being, the older motors don't have the valve reliefs cut into the pistons.