What Initial Timing Setting?

The magnum heads require less total than the older open chamber LA heads. I run 33 total. With the cam you have it's likely going to be happiest close to 20 degrees initial so you will need to use a bushing that limits the mechanical to 13-15 depending on what you settle on for initial.

Initial is determined by advancing the timing in small increments while maintaining the same idle rpm to obtain the maximum vacuum. Remember when you reduce the total mechanical in the distributor you will need to go to stiffer springs to maintain the same rpm for it being all in. If you are going to do mostly street driving shoot for the timing to be all in around 2500 rpm.