Welcome to CHR!
I agree with most all of the above – especially the information about roller cams and the input from Tech, re; compression ratio and cam timing. Is this the original engine that came in this car? If so you may be sitting on a pretty sweet deal and not realize it. The reason I ask is that you also reference “L30” – which is the 275 HP 327 from 1968 and the prize is it had 10:1 compression ratio.
The rear end that would have come with the RPO L30 engine was RPO G94-15 which is a 3.31 radio (code DD). The highest numerical ratio available from GM for the 1968 Chevelle was 3:73 so in all likelihood the 4:11 is aftermarket.
The RPO M21 is the close Muncie with a 2.20 first gear versus RPO M20 with the 2.52 low gear.
You also said she runs like a watch – so… if it were mine, I’d not mess with the cam because the heads are truly a limiting factor here. I would do the following:
1. Holley Street Dominator – like this one on ebay for $75 -
USED Holley Street Dominator intake Manifold 701R-28Z 300-28Z FREE SHIPPING
2. Remanufactured Quadrajet like this one from Summit:
https://www.summitracing.com/parts/jet-35002/overview/ with a free breathing 3” air cleaner
3. HEI from Davis Unified Ignition – like this one:
https://www.summitracing.com/parts/d...make/chevrolet
4. New 8mm wires and spark plugs – gapped at .050
5. Hooker Super Competition Headers like these -
https://www.summitracing.com/parts/h...make/chevrolet with 40 series Flowmasters and 2-1/2 inch pipe all the way back to the rear bumpers (with nice chrome or stainless tips!)
6. Set timing at 12 degrees initial and 32 degrees all in.
I know it’s a bit over your budget, but it will bring a nice smile to your face when the Quadrajet secondaries come to life. If you drive this for six months or a year and find you want a bit more pizazz you can save your money for a big block.
Remember to have fun and be sure and let us know what you come up with. Pictures are good too..
Best,
Glenn