just a thought, have you tried taking the headers completely off, then sitting the motor with mounts onto the k frame? then once successful, you might find that your passenger clearance problem may have been solved. it still might be tight and the motor may wind up where it should be. then try fitting headers onto the motor. I don't know if your headers are single tubes or welded together, but single tubes are much easier to install. even though my car is an original hemi car once I put those same valve covers on they were almost hitting the shock tower. when I need to take them off, I have to remove the distributor & hope the exhaust rockers are in the correct clearance position. I know it's a royal pain in the butt, but once the motor is in place maybe the headers, or several tubes are wrong. just my .02 cents dave ss/ea #1355

I'm seriously late to the party here, but I dealt with this years ago. I'd already read the Barton valve covers were made by Stage V. Barton rep confirmed for me.
I'd also read about cutting a T-slot (actually a V with a horizontal on top), push in, weld up on inner fender. Also welded up and redrilled shock mount holes. I had that done when I had the engine compartment painted.
Now I can slip fingers between pass-side valve cover and inner fender. Still close on heater motor, but fits.
I had this on Schumacher conversion mounts, then swapped to Devebic K-frame and original mounts. I'll look for the reference article I used.
Good luck!