Sometimes bolts that have not been moved for a quarter century can be very difficult to remove... patience is important. Whatever you do you want to try to avoid breaking the bolt off since that creates a lot of additional work drilling out the broken fastener and retapping the threads.
Liberal use of penetrating oil such as youa re using does wonders; sometimes you just need to spray the bolt and walk away for 10 minutes and tehn try it again; it takes time for the penetrating oil to do it's job. Often, it may take several applications of oil.
Using a hammer and punch to tap forcefully downward on the bolt can help to jar it loose. You do not want to damage teh head or break the bolt, but by hitting it with several moderate blows sometimes you can cause it to loosen slightly.
If you are having difficulty reaching the fastener, it may be necessary to remove whatever component(s) are in the way so you can get straight on access to the frozen bolt. it is very important that you be able to apply the force straight on and not at an angle, such as with a swivel socket or the bolt may break. A socket with an extension that is straight down on the bolt will work much better than a wrench for removing frozen bolts.
If you have access to air tools, and nothing else is working, sometimes using an air gun to remove teh fastener works when nothing else will. The air gun applies all of it's force immediately and in the correct direction; this will either break the bolt free or it will snap it off; that is why it is a measure of last resort.
Hope this helps!