The 882 head is your typical smog-era 70s head. It flows about 210 cfm on the intake side max...
An aftermarket head such as the AFR 180cc, Pro Topline 180cc, GM Vortec, Dart Iron Eagle (etc, etc) head will move much more air. The aftermarket head will also usually have a smaller, more efficient, combustion chamber increasing compression while still resisting detonation better than your old head. Plus, if you go with an aluminum aftermarket head, you've also dropped 40 pounds off the front end of the car.
As far as horsepower is concerned... I'd guess that most of the heads I mentioned above would be good for 40+ horsepower over the 882s.
Here's a desktop dyno for you... take it for what it is worth.
Using your XE262H-10 cam, guessing at 9.5:1 compression, dual plane intake, headers, open exhaust, 327 cubic inches, etc...
With the 882 heads, 9.5:1 compression... 340 horses @ 5500rpm and 389 # torque @ 3500rpm.
With the GM Vortec head (still at 9.5:1, although your compression would be a bit higher due to the smaller chamber)... 362 horses @ 5500rpm and 397 # torque @ 3500rpm.
With Edelbrock's Performer RPM head (still at 9.5:1 but see above)... 369 horses @ 5500rpm and 399 # torque @ 3500rpm.
With Pro-Topline 180cc head (compression at 9.5:1 but see above)... 382 horses @ 5500-6000rpm and 405 # torque @ 3500-4000rpm.
With AFR 180cc head (compression... yadda yadda)... 381 horsepower @ 5500-6000rpm and 404 # torque from 3500-4000rpm.
Whatddya know! The Pro-Topline beat AFR in this particular desktop dyno. Kind of a surprise actually...
So, going from your 882s to a set of Pro-Toplines shows an increase of about 40 horses and 15 # torque at peak.
- '68 Chevy Camaro (sort of done, but always subject to improvement)
- '63 Pontiac Tempest (work in progress)
- '72 Datsun 240-Z (back-burner for now)