the 65hd isnt a real cheap trans to buy either. i think you would be money ahead either with a l67 or l36. i always liked the l36 better than the l67 because of the fuel economy and still has the torque to get up and go. on my sse you get 28 because of the 3.05 gears but all the other several billion get over 30 like the bonnie SE, the base lesabres, olds 88,etc. my inlaws got 32 in their up until like 250k when the frame went. they all have 2.85 or 6. he bought a L36 2000 lesabre with like 50k probably for less than its going to cost you to put a TCM and 65hd and wiring and such.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock