Here is a link to a person that carries new Proportioning Valves for Ford, GM, and Jeep.

http://proportionvalves.com/

Here is a link to a disc/drum PV for a g-body from the http://proportionvalves.com/ site.

http://www.carolinaclassictrucks.com/78-86-malibu-PV2.html

Here is a link to a disc/disc PV for a 2nd gen f-body that looks like it will work with g-body brake lines if you convert to rear disc brakes. Please call to confirm it will bolt in.

http://www.carolinaclassictrucks.com/79-81-TransAM-PV4.html

MOPAR MASTER CYLINDER RETROFIT
I may have found a way to retrofit a 7/8Ē mopar (1993 Dodge Shadow) master to a g-body for manual brakes. I have not tested this yet to see if this works.

For the Front Brake port on the Proportion Valve:
Edelmann 258350 - Adapter-Standard To Dual Master Cylinder - 3/16" Tube - 3/8-24 Female Inverted Flare Seat x 1/2-20 Male Inverted Flare

For the Rear Brake Port on the Proportion Valve:
Edelmann 258340 - Adapter-Standard To Dual Master Cylinder - 3/16" Tube - 3/8-24 Female Inverted Flare Seat x 7/16-24 Male Inverted Flare

I donít know the length of the brake lines from the proportion valve to the mopar master, but flared lines are less than $10 each from the auto parts stores. It should be about 2 to 3 foot of line. A coat hanger can be used to find the actual length needed.

If this setup works, this will be the cheapest way to get an aluminum master cylinder retrofitted to a g-body.

Let me know if you see any issues with what has been posted above.