cast no. 302 heads for 318

I found some heads on craigslist, the seller sent me this info:
"Castings are 302's, they need to be degreased, have original valves, springs, retainers, locks, seals. came off 1986 318 out of Dodge 250 pickup. Will need new guides at least, would have them gone over, magnafluxed and such. No cracks that I was aware of, however, not my truck, buddy's truck that I tore motor down and am rebuilding with 340 J heads, and just do not need the original heads anymore."
If I got these, I'd probably have them rebuilt by a local shop and shaved down a bit for some more compression. I'm assuming that installing them doesn't require any other alterations, correct?
Anybody have any idea if doing all this is worth it? I have a 69 BCuda, 318, 4 spd. with a 8 3/4 .323 posi rear, mods include 340 cam, 340 intake mani. and holley 650 vacuum secondary carb, heddman headers, true dual exhaust with magnaflow's, and mopar eletric ignition kit.
How much power will this add approximatley? I want the car to be pretty fast (I know, fast is relative) but not crazy. It's a Daily Driver. Would it be a pretty noticable difference?
Thanks!