'33 Terraplane steering box oil seal

I finally managed to get the drop arm off the shaft with a bearing puller - with sidemounts the wheel well leaves insufficient clearance for a 4" puller necessitating drilling a hole in the well for the puller screw to go through.

Anyway, I have now found that the oil seal I got from the Railton Club is too small in diameter and too thick at 6mm. The original seal is cork with a washer. The problem is the recess that it sits in in the steering box mounting flange is only 4mm deep with a rounded shoulder bottom whereas the seal I have is 6mm thick and square shouldered. The seal face has to be flush with the face of the steering box mounting flange face otherwise it won't pull up tight against the chassis when the 3 mounting bolts for the steering box are tightened up.

So what is the solution? Do I look for a 4mm thick seal (does such exist?)or go for a new cork seal or get the flange machined to take a 6mm seal? On the latter there isn't much meat to go an extra 2mm deep without weakening it by the looks.