Personally, I wouldn't accept ANY difference in the range-check distance.

Gassers are really easy to mount. Most have 3-4 holes or mounting ears in the rear of the crankcase. These are usually threaded and designed for bolt-through from behind the firewall. That's a real pain to install and remove. I drill these out for a #8 or #10 socket-head cap screw and lockwasher, and use blind nuts behind the firewall. I've done a G-62 that way. No problems. I've never soft-mounted an engine. If the prop is balanced properly, I see no need in it.

Dr.1