If the bulkhead is truly rotting near the tabbing, then it may well separate from the tabbing quite easily. If the original tabbing itself is sound, you might consider leaving a couple of inches of the original tabbing in place, leaving a double ribbed flange into which you can insert the new/repaired bulkhead. The rotten wood can be dug/scraped out of the resulting cavity.
This would give you the ability to be certain that you've put the bulkhead back in the same place, an exisiting, original (presumably well bonded) basis for bolting into after you've retabbed over the entire assembly, with less reliance on a totally secondary bond. You could epoxy the new bulkhead into the old tabbing flange, retab, AND through bolt...
All this assuming the original tabbing is sufficient, and still well bonded to the hull.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)