Thank you all for the suggestions. It looks like 5000 is my "end" point with the caveat that things will be sounding pretty frantic by that point. I had a Series IIA LR with a low comp engine and it was obvious when it was unhappy(about 65mph downhill with a tail wind).

Location: Sunny Seattle

An 1.5in crank with Phoenix rods and slipper Pistons will do 7000 revs provided it's been balanced and put together properly. A counterbalanced crank even more.
The problem with the 1 5/16in crank set up isn't the crank, it's that standard rods aren't good for more than just over 6000 revs. However, unless you have double valve springs, it's unlikely you'll get to those speeds.
Certainly, you'd need a really good engine to pull more than 5000 revs in top on a 4.9 diff and 3.50x19s.