I use GM Parts Direct ( www.gmpartsdirect.com
). Their shipping is a bit absurd but for expensive items the base price is nice - $75.58 + $15 *&H for p/n 25523720 regullator (what my parts book shows for a 88-90 "C" engine. Appears to br held on by two screws and accessable by removing the plastic cover.
When the engine finally starts, do you get any black smoke ? - a weeping injector could do the same thing.
Once upon a time I had a '78 Sunbird with a SBC mounting dual AFBs. Only way it would start withougt a lot of cranking was after I put a "prime" button on the dash to bypass the oil pressure interlock on the fuel pump.
Push the button for for five seconds or so after sitting overnight, give a few pumps and it would crank right up.
Might be able to duplicate for a test after sitting for the five hours by turning key on, wait two seconds, turn key off and repeat five times. Then try cranking. I'd want to put a pressure guage on the system first.