The game plan is to use the stock chevy engine mounts (if it ain't broke theory) and bring a tube support from the frame
with a poly bushing that will mount thru it.
Tube cross mount with poly bushings. A tube will come from the frame mount
plate and weld to this.
The drivers side mount for the AMC 258 I-6 is smaller than the passengers mount.
Also , did you know, the engine mounts were offset 1" to the drivers side to give clearance to the driveshaft? Hmm? Hmm? Bet you didn't.
Note the darkened area of the original mount, and the extra drilled holes on that side.
The passenger side mount WILL fit on the drivers side without any drilling. (thank goodness)
I'm hypothesizing that the V8 option got equal mounts on either side.