Hi there,
I believe that the mounting distance for the Jelco 250ST (straight arm) is 210mm so it is this arm that is most suitable as a straight drop in on the Lencos. The mounting distance for the S-shaped SA-250 and the SA-750D is 214mm, so not really a straight drop in as they would be a few mm out.
The arms, when bought, come with the suitable collar which fits, with a little fettling, into the present arm pillar opening on the Lenco top plate. You would have to drill 3 small holes around the hole in order to secure the collar in place with the provided bolts. it's a pretty straightforward job.