Ouch on the idler mount! I'd bring it to an engineering shop and get them to fabricate one to specifications. It'd still be better than $1600!!!
And on the seats issue, I am out of the running. Steven, DJKOR, trentmeyer23, Ghost 5, The Viking and I (team effort) had a play around + inspected the seats and confirmed that the entire mount and rail assembly from the ACV36/MCV36 to ACV40/GSV40 models are very different. I can't even switch the rails and by the time you factor in the extra costs it's not worthwhile. :(
So HT has the 'rights' to the seats