And the finished product!!
First, I pulled out all the tubes (noting all positions) and safely removed the chassis from the old twin cab, along with the reverb tank. Then I turned the empty, old Twin cab upside-down and positioned it exactly squarely on top of the new smaller twin-head cab. I used a level to line it up perfectly on all sides as a jig to drill the new holes to mount the chassis in the new cab. Worked like a charm - surprisingly accurate (lucky?)...
While inverted, I slid the chassis into the new head-cab. The weight is lopsided, wonky, little movements at a time did the trick. Previously I had mounted the reverb tank as far towards the front of the head-cab as I could - there's about a 1" gap between the reverb tank and the 6L6 tubes (will this be a problem?). Of concern was the condition of the reverb tank cables - they are old, almost calcified. I took care to wrap and mount them snugly (but not too snug) out of the way with minimum wiggle room. Sealed it all up finished.
...And the first test run was a smashing success! ...Whew.... The first time having used my new pine 2x12 cabinet - Immediately of note was the change of my guitar tone. There is a distinct improvement in the "presence" of the sound - I have gained back a TON of high-mids and crispness that I didn't even know I was missing. The improvement is drastic.
I finished this project after midnight so I didn't get a chance to really crank it up (neighbors...) - Ill be doing so tomorrow - I'll upload a video!