for that cam in a low comp motor, I'd probably look at 18-20 initial, and 38-40 total timing

also, your dizzy advance plate will have a number stamped on it. that's the advance in distributor degrees (1/2 crank degrees). a 17 plate will show 34 degrees advance with a timing light. measure the length of the slot with some calipers, and divide by double the stamped number to get the amount you need to shorten the slot per degree (i.e. if you have a 15 plate, and the slot is .3" long, you need to shorten the slot .01" per degree, or .1" if you want to reduce the advance from 30 degrees to 20.)

when you weld up the slot, weld up the inside. this adds preload to the springs, which will cause the advance to start in at a higher RPM range. most stock distributors I've played with, the light spring is so light, it starts advancing at ~700 RPM or less. if your neutral idle is above where the mechanical advance starts advancing, you'll get excessive RPM drop in gear and an unstable idle. I like setting up my mechanical to start coming in at ~1000 RPM