I had borrowed an engine picker in order to pull the engine and take it back to the machine shop, but the machinist is away until September. (The joys of dealing with a rural ''Mom and Pop'' small business)
My buddy had a look at my engine mounts when he dropped off the picker, and said that perhaps we could cure both the firewall and floorboard problems by just mounting the engine and trans a couple of inches lower. I think he may be right... there's a lot of clearance between the oil pan and cross-member.
Now that I have removed the cab again, I can also see that the trans yoke is a couple of inches higher than the driveshaft center bearing. It's amazing what a fresh perspective can do.