I have my 350 in my 72 Chevy pickup timed about the same. I pull a 5000 lb trailer with it once or twice a year. This year it was knocking while pulling up a grade. Outside temperatures were close to 100. I put in 92 octane gas so with the regular in the tank it was about 87. The store were we go fishing only sells 92 so I filled it with that for the rest of the trip. It solved the knocking problem and I was getting 15.5 mpg. It runs fine on regular under normal conditions but I have never had as good mpg with pure regular. Part of it might be the fact that I was only going in the 35 to 45 mph range most of the time.
Vacuum is ported. I tried manifold but it won't start at times. It just locks up. I read 16" at cruise on the level (no trailer) and 18" at idle. Downhill I can get it to 21-22".