I'm leaning towards the stainless option
Did you just replace the barrel assembly:
Or the entire master cylinder assembly:
Ime, 'fraid this can't be pre-planned, depends what you find when you dismantle.
Worst-case, you find whoever worked on either master cylinder last didn't grease either the locking grub-screw or the master cylinder thread into the ally mounting casting, and these are now corroded together; if any fluid has leaked from the master cylinder, the corrosion'll be even worse.
Note it isn't always impossible to separate the parts, I've had 'em separate with only moderate force after only being submerged for spark-erosion of the grub-screw. However, I didn't have to travel very far to the company doing the spark-erosion ... if you do, it might simply be cheaper to pay for a complete new master cylinder, with both stainless cylinder and piston.
If you do buy either a complete new assembly with both stainless cylinder and piston, or just a new stainless cylinder - as they also aren't made by any part of "Lockheed" - ime I advise against simply assuming it's a bolt-on assembly/part ... faults I know of on pattern stainless master cylinders:-
. cylinders not honed after ports to reservoir drilled - sharp edge of holes inside cylinder nick seal;
. neither master cylinder-to-mounting thread nor locking grub-screw greased;
. master cylinder not adjusted correctly in mounting - Lockheed manual
. locking grub-screw not stainless ...
Will do the same for the front brake, and while I'm at it, rebuild the caliper as well with stainless pistons
Plus braided hoses with stainless end fittings replacing both steel pipes and unknown-age rubber hoses? I appreciate not cheap but fwiw the first first braided hoses I made well over thirty years ago are still indistinguishable from brand-new.