Thanks for info, Fred. It's quite possible I've been reading this over in
another forum (LS1Tech). I've discussed this with Dave at Yank and
his opinion is this (quoted verbatim):

"... the poly mount will break the tail housing over time and is it not
needed unless the frame is tied together with a roll cage."

and from Chuck at FLT Transmissions:

"I think using the entire kit you'll be ok, most guys just use the trans
mount which I think causes problems at times because the motor is
moving more than the trans. I don't hear about the LT1 tails cracking
like the LS1 tails for some reason, maybe because the difference in size.
The only thing I don't like about them in a dailey driver is that
they can cause excessive vibration that you might not like."

(the "entire kit" refers to the complete poly engine/tranny kit I had
inquired about).