I'm a little confused as to exactly what your question is...sorry.
If you are asking for a converter recommendation, then more info would be needed (gear ratio, carb, car weight, tire height, camshaft, rpm range, etc., etc., etc...). If you talk to a good converter shop they will ask you questions about your entire combination and help you narrow things down. I used Dynamic Converters in Newark, DE (302) 454-9203 and have been very pleased with mine.
You are correct. The number of splines on the input shaft of a 65 727 trans is different than a later 727 trans. The later converters appear to slide on but will not properly engage the pump which can trash the trans. Make sure you are getting a converter for a 65 727. Up to 1965 has a cable shift and the old style converter (someone please correct me if I'm wrong). 1966 started the lever shift style and the later style of converters. The converters are not exchangeable between earlier and later years
I've got a 392 ci Poly stroker that dyno'd @ 405 HP & 472 TQ. I ended up with a 3,000 rpm stall, but keep in mind that my converter would likely stall higher behind your motor, and differently behind another motor. That's whay it's important to talk to the experts that build and design these things for a living. I swear they are partially filled with voodoo!
65 Coronet 500 'vert http://goo.gl/6xUS1
73 Dart Sport, 05 Ram 2500 CTD, 4WD, QC, LB, Laramie14 Avenger R/T,
15 Durango R/T, 21 Model T Depot Hack