They may well be corroded into place via bimetallic corrosion. Any time stainless and aluminum are together for a long time you can get this. Try heat.
When you do manage to budge them, consider replacing them with solid SS pins with cotter pins both ends. While you're at it, push on a Delrin or nylon washer onto each end before fitting the cotter pin. The fittings you have were provided by Kenyon originally and represent rather old-school thinking. They're fine so long as they're in good condition; but remember all welds represent some minor level of metallurgic dissimilarity and after 40-odd years they may not be as reliable as when new. Remember that the sheave pins don't have to be tight in position, meaning side-to-side. They just have to not move up and down or fore and aft. If they were an extra 1/8" or even 1/4" between the cotter pins it would not be a catastrophe - actually might be better, allowing some little movement, to avoid the corrosion from being fixed in place.
If you reuse what you have, follow the tried-and-true, industry-secret procedure of putting white electrical tape on the back of the plate where it will go flush against the spar, to metallurgically insulate the two metals. Trim it around the edges (not too close). You'll be surprised how well this works.
Use a Delrin washer for the other side.
No matter what you do, you're very likely to have to do some restorative surgery to the surrounding aluminum, which I expect will be well eaten by the stainless. Sand and file well - be sure to get rid of ALL traces from steel file or wire brush (DO NOT USE BRASS OR BRONZE!) - use epoxy with aluminum powder, and sand to your liking. And don't put it together again without a liberal coating of Tef-Gel on everything. Good luck!