No problem.
The timing #'s sound right. We were at 24* prior to adding an intercooler. I have a firend w/ a 406 SBC running upper 20's on pump gas with his blowthrough system.
You hit the nail on the head as for timing/detonation. My best advice is to stay conservative.
Have a couple sets of NGK's on hand (we used 9's hot-air and 8's with cooler temps) and you should be able to read the ground strap. Pull all 8, keep a real close eye on the porcelean and sneak up on it. Tuning via mph is still a good process if you know your car and are willing to weather correct.