I got that, what I was hung up on was the larger TC being mandatory for the HD and I was under the impression (until your previous post) that it was mandatory both ways, that only the HD could use the larger TC. I see cars with 258mm TC but without SC so that is what had me confused.

I actually don't want an HD, I don't need it for this build and its just additional cost and complexity (having to drill the flexplate, different axle inners, etc). Looking at trans tags for 1998, the two best matches for subset of [comfort, gears, tire size, weight, stall] are probably the 8FFB for the 3.8 NA Park Avenue and the 8TNB for the 3.8 NA Regal. The Park Avenue is probably the better fit, but it shows the large TC, hence my questions--is this an HD trans on an NA motor, or is it the stronger TC on a regular 4T65E.

I think its just a larger TC. Strength doesn't play a factor, but probably stall speed would.

According to this page the 8FFB from the Park Avenue is non-HD (text search for 8FFB). That would mean the TC is the only difference between the luxury Park Avenue and the cheaper cars that used the 8TNB. Might be comfort factor--longer spoolup time, different stall, more PWM slipping programmed into the TCU requiring larger TCC clutch or improved heat shedding or something, etc.