I assume that this is a Lucas 11AC setup or did it start life as a generator system? I have extensive experience with old Lucas systems and can help. You may also have a 3AW warning light assimilator, either on the firewall or maybe behind the driver kickpanel.

The 10SI is internally regulated, so the trunk mounted VR would do nothing after conversion. Aside from the large B+ lead, there are two 1/4" terminals, labeled 1 and 2. The "1" lead is D+, and the "2" lead is voltage sense. The "1" lead must be used to ground the panel light, as it supplies bootstrap current to the field. The 2 lead can usually be attached directly to the big B+ terminal, but if you find that charge voltage is inadequate, you would need to run it back to the rear of the car for voltage sense.

I can show you how to do all this, but again, you'll need to post some photos. I'll need a photo of that VR to identify it, and I'll also need a photo of the rear of the old alternator. Year of the RR would also help.