Well.. what will you be changing, or not changing?
You've already got headers? Cool. One less thing to worry about, just get that mini starter.
727's have a larger loss just because everything is bigger. More rotating mass = more parasitic loss. 904's are lighter and only take about 25HP to turn as opposed to about 45HP for the 727, IIRC. Here's a link for the flexplate you were asking about: http://www.summitracing.com/parts/bmm-10239/overview/make/dodge