Careful Jason, this is a real snowball....
I remember Chevrolet by the Numbers having a good discussion on the difference between various heads. I am looking for something online that gives the same basic tutorial.
Looking at 67 Chevrolet data from GM (someone correct me if I get any of this wrong), the differences between a 327/210 and 327/275 (L30) (engines only at this point) were the heads, the crank type, and the intake and carb. The camshaft, pistons, rockers, and other basic components were the same.
Heads - the L30 got the same heads as the 350/295 (L48), with 1.94 intake valves and 1.5 exhaust vs. 1.72/1.5 in the heads you have. The combustion chamber is also smaller, by 11 ccs or so. An L30 had an advertised compression ratio of 10.0:1, vs. the base 8.75:1. I am not up on the runner volumes of 291 heads vs. the 775s, but I'm sure the 291s flow more volume in concert with the 4bbl intake.
Crank - The L30 got a forged steel crank, while the base crank was cast nodular iron.
Intake and carb - L30 used a cast iron 4bbl intake and Q-Jet vs. base engines cast iron 2bbl intake and 2 bbl carb.
The output curves between the L30 and base motor are fairly similar in shape, as you'd expect. L30 makes peaks of 275 HP @ 4800 and 355 ft lb @ 3200 base makes 210 HP @ 4200 and 320 ft lb @ 2400. But the L30 has a "broader" torque curve, making more than 300 ft lbs from 1200 to 4800, while the base only makes more than 300 from 1600 to 3200. So, all around, the L30 with it's better heads and intake is a stronger engine.
The 291s Plum linked to would be "right" for a 327/275.
All that said, the single biggest change in off the line performance using "67 parts" would be to put a 3-speed or 4-speed back in it (I know, just what you want to hear
). The 1.76 first gear of the Glide just kills performance off the line, compared to a 3-speed with 2.54 1st or 4-speed with 2.54 or 2.52 1st. I know that's not realistic at this point, but I'm just saying...
Again, for 67 vintage OE parts for a 327, it's hard to beat the fuelie/camel hump/double humps (1.94 intakes, not 2.02 that came in some of the same heads). If you get a set of heads that haven't had hardened valve seats (for unleaded gas) that may need to be addressed. And it's tough to escape the fact that any used heads like this have been around for 40 plus years, possibly on and off various motors... you can get quite a bit of money wrapped up in something that was good in its day, but there are better solutions out now.
Something like the Iron Eagles JimM was selling, except if you don't change the cam, you probably want the smaller valves.
It's all about working together as a package. And it can definitely snowball....