If you have to buy a new flex plate, I would just buy the B&M one for the 360. That way you don't have to run any weights and it opens up some choices on convertors.
+1 Here is the one that I am installing for my 318-360 swap. I am in the same situation as you, but my engine builder balanced my crank with the flex plate and new damper installed. http://www.summitracing.com/parts/BMM-10239/