You could try some penetrating oil at the collar of the distributor applied over a few days, but it may be that after lack of use the collar is a bit stuck, so you may not have fully slackened it. The housing is aluminium, and the dissy (probably) steel, so you could try a little heat on the collar gently, but note that there are petrol lines around there also, so don't use an indescriminate naked flame. I would expend some effort with penetrating oil and attempting to move it back and forth before removing the entire housing. I think you'll find it will move with a little preparation.
Apparently a 50/50 mix of ATF and acetone makes the best penetrating oil. I've also had success with old brake fluid (which I think JA recommended).
(Don't beat on it with a hammer, tho).
JH