I like budget upgrades and the gt40 heads seem like a good route, as long as you can get them cheap. If they need work, then you'll likely have as much in them as an entry-level set of aluminum heads. Keep in mind that you'll already need to replace the valve springs, unless they've already been upgraded and will support your cam.
You can also use a gt40/explorer intake, but you'll probably want to upgrade the throttle body and egr spacer. I can't recall whether you can use the oem one, but I'm thinking it won't work with the fox intake tube/air box.
I upgraded my 88gt with a set of ported c9 iron heads (similar in specs to gt40), explorer intake, sve tb & spacer, b303 cam, 1.7 roller rockers, and 24lb injectors/maf. I would be very surprised if it isn't making a little over 300hp to the wheels. I doubt you would see 300 hp without a cam swap and/or 1.7 ratio rocker arms. I'm not sure if 24 lb injectors are needed, but it's more of a peace of mind deal. I don't want to worry about going lean at high rpm. Forgot to mention I also have aftermarket exhaust, no cats, BBK intake and an electric fan. With just gt40 heads, tb/spacer, gt40 intake, 1.7 rockers, better exhaust, I would be happy with 275ish. With a lower gear, you would really feel a difference as far as seat-of-the-pants goes.
