Go with as long a traveler as will fit. You want to be able to dump the traveler in puffs, without having to dump the sheet... Although a vang will keep the boom from lifting, a traveler is still more betterer...
How about a complete kit? Probably overkill though.
Catalina Direct: C-250, CP-25, C-27, Traveler Upgrade For End Boom Sheeting
You'll have to figure out how you are mounting it, if the tiller has to go under it (likely)... you'll have very little room to lift the tiller... and there is the whole how do you reinforce, and raise where it'll be located. That's one of the downsides of gudgeons and pintals, mounted on the stern, having a stern mounted traveler isn't as practical... I know mid-cockpit stinks for getting around, but it is actually the best place to keep the skipper's eye's forward, and can be adjusted with a free hand... Like the J/24, J/22, Capri 25, Capri 22, B25, and most racers in that size. If you are cruising though, then this position isn't ideal.