I would hesitate cutting off the studs until every other try has been exhausted....having something to heat and twist and bang on is preferable to nothing.
If the replacement part going in will use holes sufficiently apart, then fine, use them or cut them off.
I looked at doing this job on my boat in order to get the log out if the keel. After 6 months of research and calling all over the country for advice. The vast majority of yards and individuals that did it, said either, they would never do it again or resulted in tearing the majority of the inside over the keel out and cutting large sections of the keel away on the exterior to access the log and free it up.
But universally they all described both ends, the stuffing box flange and the bearing carrier being screwed on to the shaft log.
To just get the packing gland bulkhead plate off isn't usually a huge job. Many times after getting the hanger bolts out, it will unscrew from the log without much fuss. Now if there is no clearance, cut the old one up and either modify the new or dig a trench so the new can twist on. Last resort fo me, knowing 20 more years out of my boat might be it......get rid of the threads and slip/epoxy fit the new one so it can just be pushed on.
Because pleasure boating radically changed in the last 50 years or so.....this is similar to the shipwrights in my neck of the woods.
Home - Port Townsend Shipwrights Coop
About Port Townsend Shipwrights Co-Op
The talented crew of the Port Townsend Shipwrights Co-Op is well-known for their traditional woodworking skills: from fine interior joinery on sail and motor yachts to major structural restoration on commercial fishing vessels. Their attention to detail and depth of knowledge insures your modern or classic boat will leave the shop in superior condition. PTSC also specializes in new construction, metal fabrication and fiberglass. Design and installation of Electrical – marine electronics and navigation systems, Mechanical – engines, propulsion, fuel and drive, Systems – heating, cooling, plumbing, steering, refrigeration and hydraulics, and Spars and Rigging.