Don't waste your time looking for or attempting to use original specs. They hold no value anymore. Even if you do learn the correct jet sizes and tune-up settings in a service manual or other old book, they would be entirely incorrect/obsolete. No jet spec or other carb setting from 1968 will work as designed when flowing today's fuels, which are drastically different than they were 45 years ago. Same holds true with base engine timing, distributor curve tuning, compression ratio, etc...

With today's fuels, EVERYTHING has changed. So you have to ignore most original tune-up specs and find your own.

Anyone who attempts to build and tune up these cars using 1968 service manual specs is going to leave a LOT of performance/fuel economy/drivability potential on the table...but the car will indeed "run and drive" a least for a while if all you want to do is get it on the road to sell it or something. At best it runs below potential. At worst it slowly self-destructs.

I agree with OHCMarc's suggestion, Cilff Ruggles is a Qjet specialist who knows how to get you pretty close with modern fuels, and for your particular engine's level of performance, from a carb perspective anyway. Then after rebuilding to one of his recommended recipies, in order to get it to run the best it can, you will still need to fine tune it from there and re-tune the timing curve, ideally with the assistance of a dyno and an O2 sensor.

Cliff writes a real nice book with jetting, mods, and initial setup suggestions for quadrajets mounted on various engine power output levels running on modern fuels. "How to rebuild and modify Rochester Quadrajet Carburetors." Available all over the internet, it's a fabulous resource for this type of thing.