360 build for dart swinger

Do you have the heads off the 10k mile motor? What are the casting numbers of on them? Just FYI all pre-magnum 360 heads are basically the same as 340 4bbl heads accept 360s (and late 340s) had 1.88" intake valves. Don't go chasing J or X heads because a 75' 360 will likely be as good of a starting point if the head is healthy, as you noted the exhaust seats like to get pounded in. If I may interject on heads, if you don't have a good use-able pair look to new, either aluminum, EQ Magnum, or LA-X heads. I prefer aluminum because they are lighter, easier to port and are easier to repair. Stock irons are great and can make good power accept when they need seats replaces, new guides, valves, port work, this adds up so do your homework and math on the direction you take. Since we don't know your piston height and possible piston cc it is hard to guess what your static compression will be with whatever head and gasket combo.

A 350-400hp street 360 is obtainable easily with the correct matching parts and that is the key, parts that are designed to work with your combo.

Rumblefish is steering you in the right direction.