I think my C3 is shifting too early.  First to second kicks in at 9 or 10 MPH and second to third at about 20-22 mph. Does any one know what is standard for these 3 speed turbo 350's?

I tried adjusting the vacuum valve on the back of the transmission, but I don't think mine has an adjusting screw inside.

Any help would be appreciated.

Does seem a bit low. Is the carburetor running on 2 bbls or 4 at shift points? 

two barrel

Depending on the amount of throttle you're giving it, the "normal" shift points are 1-2 @ 12-18mph, the 2-3 at 25-28mph. Light throttle shifts will be earlier, heavy throttle shifts later, of course. You are correct in noting that not all vacuum modulators are adjustable. I would suggest installing one that is, and adjust it to shift where you want it to. Check the available vacuum at the modulator also, and determine from where on the engine that vacuum is originating. Sometimes, and orifice is needed in the vac line to delay upshift points. In the old days, we would use the end of a glass fuse, and drill a small hole in it, then insert it into the vac line. 

Does it shift too early under heavier throttle also? If so, then I would suspect a governor issue more than a vacuum issue. Is this something that has just started, or have you had this early shift issue for a while?

Be careful with the adjustment. The screw only has a certain amount of adjustment before it rips the rubber diaphragm. One or two turns in/out will usually get you where you want to be. I wouldn't go more than 4 rounds....if it needs more than that, the modulator ain't the problem.

Thanks Joel, just what I was looking for.