Mick, personally I wouldn't want to rely on the springs for clearance as sooner or later you're bound to go over a speed hump a little fast and there goes your sump.
The lower part of the engine mount on a Harrison, the fabricated section that wraps over the chassis rail, have you considered making that piece "taller"? It's an upside down U where the bolt goes through into the chassis rail... if that was 20 or 30mm higher it would raise the engine. How much clearance is there between the engine/trans and the firewall/tunnel currently?
Have you spoken to Warwick about this?