I had a similar problem recently with the very same engine in my '69 Camaro. Every time I was checking the vacuum, and adjusting the idle screws on the carb, I was also disconnecting the hose from the vac. adv. on the distributor. Vac readings were fine, but it still felt like a vac leak. I checked everything, several times. Well, almost everything, it ended up being a faulty advance module on the distributor. I found out once I put a 'T' in the line for my vac gage.