Well, do you use water as engine coolant? If so, then you may have installed the HG incorrectly. I've done it before where I put too much oil on the head studs causing it to make the head lift a little. When you took your engine apart, did you send the head and block in to get milled? If not, then there won't be a perfect seal and you'll need to take it all apart and do that.
On an older engine, like the one you have, it has probably been warped on both ends a little. Getting it milled will also raise your compression so you will get a little more HP out of it. Nothing crazy though.
Use a 2-layer metal gasket, not a cheap paper one. That with some APR head studs should to the trick.
