Hi, welcome and good luck with your first build!
I kill two birds with this one.
I cut off those hard plastic hand grips and use foam ones, but they are longer than I need so with what is left I use it under the tank.
Unless your seat tube is too fat for a good fit, don't put anything there other than maybe a little locktight.
And pinning it won't counter the twist, that is what your front mount is for.
If it all possible use the dual stud mount so you have a stud on the right side of the front tube, that will counter the twist the best and it is a much longer lever than a pin at the pivot point.
As you read through the myriad of tensioner ideas here you will find one common mantra of mine, if it is at all possible DON'T USE ONE ON THE DRIVE SIDE!
If chains won't match make the pedal side long and put it there.
You won't need to worry about rubber or pins or anything, the pedal side is human power and seldom used.
You don't need to use the big ugly hunk of metal tensioner for the pedal side either, a nice little one like this works just dandy.
All my pedal side needed after getting the motor side right was a single 1/2 link.
I hope that helps ;-}