Thank you very much. Unless I can make my own fuel I guess I will sell the 670's
Hang on there zzomby, don't make any hasty decisions until you do a little more research. As far as physical installation of the 670's on your 389 there are two issues you might run into.
* One is that "at some point" (I forget when exactly) Pontiac changed the inclination angle of the valves slightly. That means the location and shape of the valve relief pockets in the piston tops might not line up correctly with the valves. There are some "Frankenstein looking" aftermarket pistons out there that actually have valve relief's in the for both angles (two complete sets of reliefs in each piston) but they aren't good for a performance build.
* Two is that the valve clearance notches in the tops of the cylinder bores might not be right for the 670's.
Both those problems are (relatively) easy to solve though and not a reason to toss the heads.
As far as compression, yes you need to be careful there but in general putting "small chamber" 72 cc heads on a smaller displacement engine results in less compression, not more. (Compression goes up when displacement goes up, head chamber volume remaining constant).
What you'd need to do would be to get actual measurements for your engine including bore, stroke, piston deck clearance, and compressed head gasket thickness then calculate where you'd be on compression with the 670's. You want to measure the chamber volume on them too so you'll know what they really are. Standard dimensions for a 389 are 4.06250 bore by 3.750 stroke but if yours has been overbored by a previous rebuild that will figure into the calculations.
(I'd run the calculations for you through my spreadsheet using "normal" dimensions but I'm away from home right now and don't have it with me.)
Larger valves are a good thing.
Specific to the 670 heads beside the fact that they've got the big valves and screw-in rocker studs, is that they're the last "closed chamber" performance head Pontiac made. They are very popular with some engine guys, but personally I've never quite understood why. They do flow well for D-port heads, but those closed chambers hurt combustion efficiency meaning that they need more ignition advance to be "happy" than an open chamber head does. More advance means more opportunity for "negative work" in the cylinder -- you're building combustion pressure before piston TDC and that's working against making power. That closed chamber though means they've got a huge quench pad area and that does tend to promote lots of turbulence in the mixture and that's a very good thing for helping to avoid detonation.
Compression ratio and related topics tends to polarize people into two camps. Those who tend to push compression ratio right to the razor's edge and those who shy away from doing that. Both sides have some very good arguments and some folks on both sides are very successfuly at building engines that live and also make significant power. I tend to be cautious with it myself, but I also try to be objective and sort through the 'chatter' to understand the truth underneath it all.
There's more than one way to build a Pontiac that makes power.