Not normal. It should be happy with more timing at light throttle. For example a stock 1968 340 distributor would have advanced 22 to 24 degrees at 2400 rpm, plus whatever the initial timing was spec'd at. For cruising it would have 17 more degrees as long as there was at least 10 inches of manifold vacuum. So that's at least 39 degrees while cruising at 50 mph or so.
How are you measuring the timing?
What distributor is that? Are you going by info in an instruction manual? It might be giving you distributor (cam) degrees. My advice is measure and plot out the timing curve. Then measure when the vac advance starts and stops as well as how much it adds.
