What is the best way to get an overdrive auto in a 67 Barracuda?

The 4L80E can be converted to manual shift with a trans brake etc. with a valve body that Extreme offers for about a thousand bucks otherwise you can buy a stand alone setup for about 500. I probably can get a 200R4 that will hold up Extreme has one they advertise to hold 700 hp for 2300.00 but add the cross-member, bell housing, adapter, converter, then compare to the cost of the GV behind my 727, the economics part of me is asking if the extra 200-300 rpm difference you get from a .78 OD vs a .67 is worth the extra 2k bucks?