Hi Jim,
I'm not clear on your question, yes the cable can be replaced from underneath.
Regarding the clutch I've spoken to the guys at Thunder Road Speed shop in Woking England (my car is currently at their shop with it's interior out). Apparently it's a big job, they allow 2.5 days for a clutch job. A major factor is if your car has a removable crossmember for the gearbox. If it does then the job is a little easier, but the transmission tunnel and seats still need to come out to get to the propshaft, gearstick etc. If you have not got the removable crossmember (bad news), the interior has to come out and so does the engine and gearbox. Thunder Road have three or four MkIV cars through their workshop each month and really know the model well and are very helpful if you give them a call. Also Alan Faulkner-Stevens knows the model very well and may jump back into this thread.
Alec at Thunder Road is happy to help answer your questions if you want to call him or email. Send me an email john@moonlightmedia.co.uk and I will give you Alec's email and their phone number is +44 (0)1483763399