I'm having issues I am doing a disc brake upgrade on my 68 ss it was factory with power drum brakes. I have installed all the lower stuff supplied from a 69 camaro but was wondering what type of proporting valve to use. I was told the 70-81 valve was my best option or the original one would involve me also using the metering block. AS I already have a busy engine compartment (lots of flash) I'd like to use just the proportioning valve only. If suggestions on a new one or used 1 I just need something Thanks for the help.

You might find this interesting. Out of Proportion (http://www.inlinetube.com/articles/Out%20of%20Proportion.htm)

http://www.inlinetube.com/images/Prop%20valve%20article/DSCN0029.JPG

I have the exact same car. Like you, it had power drum brakes. I upgraded to a SSBC front disc conversion and used an adjustable valve (included in their kit) like shown in the pic. After installation and break-in, I took the car to a deserted road and carefully made panic stops and adjusted the valve so that the rear wheels didn't lock up before the fronts.