the flex plate would be the easiest,and cheapest way to do it wit hthe older 904. as for puling hills,,you want that 3;21 or 3;23 or 3;55 you surely dont want 2;72 rear gears for hills,,
I had 2.94's with 28" tires on my 69 Fury with a stock 360, a 440 thermoquad, and a stock 318 trans and converter. Has plenty of torque, and thats a c-body.