Cortina diffs ration help

[locosaki]

I've already posted in the general section but thought it may get a reply here.

I'm running a ZX9 through a cortina (Salisbury) 4.11 diff, accelerates like a b*stard but tops out at 96mph. I can get a 3.75 from a 2L, but that only takes me to 105, and a 3.5 (£300 from Lotus Cortina spares) to 115mph.

Anybody know if there are any available with a lower ratio? Or do I have to get me an Escort (English axle)

BTW, I'm already running 15" rims.