Hey Zedder,
I have the same setup as you, but my 4 post lift and highlift conversion are on the opposite side. Like your garage, my doors are tight to the exterior walls. I'm planning to do another highlift and 4 post lift on the other side eventually.
You do have a few options...
You could side mount your existing garage door opener.
Or you can remove part of the drywall on the exterior wall and check to see if you have enough clearance to fit the jackshaft opener in the cavity.
Last option, is to mount the existing opener on the ceiling horizontal, front facing down. You'll need to build a mount to bring it down off the ceiling to fit the opener into the shaft. The highlift conversion on the other door would have to be about 1 foot lower than the current highlift for clearance for the new jackshaft opener. Which will be mounted about 1 foot lower than the current one. I hope this setup makes sense?