Well, any of the cantilever mounts are going to be high mounts. If you put one of these high mounts on your ARMS #38std, your Aimpoint will almost certainly sit too high for your liking. It will absolutley be too high to co-witness.

What Aimpoint mount are you using now? What I would siggest is dropping the #38std, get a seperate BUIS (like the ARMS #40 is you like the flip up one on your #38), get a cantilever mount and attach it direct to the flat top.

Cantilever mounts I am aware of are

ARMS #22M68 with extended spacer
LaRue Cantilever mount
GG&G Cantilever mount
Knight Armament Offset mount
MI Cantilever mount (much like the GG&G)

If, for some reason, you like your #38std sleeve, just get the #38ex sleeve instead. Its the same thing, but gives you a longer rail. If you decide to do this, be careful, the #38ex sleeve will not fit over the oval M4 style fatter handguards with modifying the handguards. This is yet another reason I think you should just drop the sleeve, get a seperate BUIS and use the Cantilever mount right on the flat top.