I put a T5 in my 3600 with a .72 overdrive.
I also changed out the rear axle for a better ratio. My single driveshaft that I had made was not too long, according to a very reputable driveshaft building company here in Norcal. Before I gave them the measurements, they told me that they would refuse to make me a driveshaft if it was too long. I believe it came in about 2.5 inches under the max. It also had to be a 4.0" diameter.
I chose a rear end with 3.73 gearing. I haven't driven it yet, but that was where the math took me with the 27" tire height that I wanted. A couple others here have 3600's who have done the same thing.
I see you have good taste in English cars too
