Jay,
I have the first 6 speed in my shop. The adaptor plate we use on our 5 speed fits and the starter will remain the same. The nose of the input shaft is further into the bellhousing and the diameter and number of splines are different from the 5 speed to 6 speed (bummer - the 4 leaf clover has deserted me for this one anyway !!!).
The overall size is comperable but the shape is better for strength.
Our conversion will fit directly into your car , pick up on your gearbox mounts and we will include all the parts that are different to the parts you have at NO cost on EXCHANGE. I mean such things as the gearchange, mounts etc. etc.
Gordon is trying to price and get availability of the boxes in the US but I can get them in Europe. Hopefully he can source them at a good price in the US.
As soon as the pricing and the conversion is completed I will post and I hope to complete this work in the next few months. This time of year is very busy with F1 and the rest of the racing going on.
Keep up the good work on the car,
Best wishes,
Robert