Problem for most of us re a Ricardo 6-speed is -A- the sheer expense and -B- the box is much heavier-duty and bulkier than the ZF so requires either mounting the new transaxle higher so the lower rear a-arms mount under the gearbox, or spread the lower rear subframes & shorten the lower rear a-arms, then complet- ely re-do the rear suspension geometry. I doubt if G. Dallara would be interested in redoing his work of 1970 just for a few of us that want a pricy 6th gear. If I did, I'd go for Butfoy's 6-speed ZF and keep stock rear suspension.
Tom, since you've already gone from 15" to 17/18" wheels, your brake discs can also increase by 2"OD. That would massively increase the brake system leverage especially the fronts, and will noticeably upgrade your stopping power without increasing required leg power. The stock booster will provide enough assist.