Looks pretty light-duty. Lots of A/C tube is thin walled. How about making an expanding mandrel to fit? Make one with a long taper, like a Morse taper or Braun and Sharpe. Might require a few practices on scrap and re-positioning a few times. Automotive tail pipe expander kits start at 1...
Have you gone thru the tec L head service manual and looked at the governor set-up? Most engines I get like yours, the previous owner has changed the governor in an effort to cure an ill. The lever/stem can be out of position, the linkage bent, and the spring might need replacing.
It has really helped me to clamp the bead with a padded-jaw, non-marring Vice Grip. It captures the tire bead and prevents me from making several trips around the wheel.
That and some baby powder for the tube and tire.
Good ideas here, thanks everyone.