My distributor had all kinds of thing wrong with it. When I was setting the timing I didn't have a timing curve at all even with vacuum advance. I check the vacuum advance by sucking air. It felt like I was sucking air through a straw so I decided to just take it out. The mechanical advance springs were so worn that the weight move up and down and left in right just by turning the distributor. It only had one plastic circle thing on there two. I took the old one from my 305 and I tell you what this 307 felt like a whole new motor. I set the initial timing at about 8 degrees, I haven't got around to buy timing tape yet so finding out what the total timing so I can adjust that but the total timing I'm going to try 36 degrees on this 307 sbc. I bought this 307 with a th350 and replaced my cracked 305 block and stock 4 speed manual. Here is a video of right after I put the other distributor in.
http://www.youtube.com/watch?v=dNrZonPzpMA