4T60e(HD)? I didn't think the 4t60 came HD? That would've been in Riveria's? (Still learning)
(Disclaimer): this is mostly just by observation of Regals/Olds 88's that i noticed.... In the 95 and older 3800, there are two motor mounts, one in the front of the engine and one in the back that attaches to the transmission which is a two piece. The 96 & newer have one engine mount under the center of the engine and this bracket that is one piece that attaches the trans end to the back of the block. I had GM dealer run a 'where used', this part number and they said this bracket is only used in 96 Buick regal. Not saying their book is correct.
Now i'm looking through my pics from junkyard. Looks like where the bracket attaches is different, but would've worked?
96 Regal Bracket
Pre 96 4t60e
Looks like they attach to different holes on the trans, one is between trans and block the other is above the trans (96). The 96 really made it difficult to get the the last bolt on the trans/block. Dont know if the others would be easier? I went with the 96 because I knew it would work from SeriesII and I was able to buy it.