97 is the pivot year for the 60 65 transition. 97 N/a still have 60'* and boosted guys have the 65-HD. In 98 all 3.8 bonne'* had 65'* non-HD for na and HD for charged. 65 are superior for the most part because of the 9 vane pump in the 65 as compared to the 7 in 60'*.
As for difference in non-hd to HD the difference is the differential and housing plus a different passenger side axle. TO upgrade the 60 is a little more involved but upgrading a 65 is pretty simple. If you are going to go HD I'd spend a little more time and money and get yourself a 98+ 4t65e-HD for a H body.
97SE-180*stat, Gutted Air Box, Green Air Filter, Rear Air Shocks
^Future^- HV3, Stage 2 Heads, L26 Upper, P&P LIM, LS6 VS, LS1 Retainers, N* TB, LQ4 Maf, CAI, 65-HD Trans, GT2 cam, SSEi sway ES links