Hello Tony:
I too have been doing alot of homework on the Jaguar Series 2 rear gear change process. I would assume this info does apply to your series 1 rear, but an not knowledgable in those particulars. I have an ERA with 4.11 rear gears. It is a real slingshot off the line. However at 3000 RPM in 4th I am doing 61 MPH so my top end suffers.
Now for the process that I have found out about...
The changing of the rear gears is bench work. It requires dial indicators and some finesse to do the set up. Thus, virtually no one who does this work will do it in-situ. Translated... The entire rear suspension must come down and out.
Second, the center section of the rear suspension needs to be benched so the rear suspension must be dis-assembled. During the disassembly, you can, in theory, replace worn parts or those that need refreshing.
I hear that once you have the center section out, the best process is to totally replace it with a center section that already contains the gearing and LSD that you desire. You just keep your old one as a spare. Thus what you are looking for then is a back up center section from a similar JAG rear that you have now.
There is a firm called Reider Racing
www.reiderracing.com. They specialize in rebuilding such JAG center sections and rebuilding the LSD and setting it all up. Certain Cobra firms use Reider to do all their work. The typical going rate today for a junkyard center section from a correct Jag Series 2 is now in the $500-700 range without shipping costs, i dunno if your Series 1 would be more or less, probably the former. I have no idea what the fee is that Reider would charge to deinstall and reinstall correct gearing. They can tell you that. I do not wish to speak for them as prices I have are for series 2 not 1.
So the course of action as I see it is.... one of two choices. Upgrade your current center section or get a second and have the gears and LSD made right. The bene of the second choice is that you have a spare on the bench and you can shorten downtime of the car by having it all ready prior to starting the rear suspension pull.
I am still, personally, very very torn over doing this procedure. My 4.11 rear gears make my Cobra a rocketship from standing start and although my engine is busy at 60 MPH plus and I have given up high end top speed, the car is a heck of a lot of fun to drive.
My other option is to go with a 5 speed trannie and pull out the close ratio Toploader. I have not made up my mind. My 428 is a torque monster, especially between 2800 and 5200. Nearly a flat torque curve at 435-445 ft/lbs, as currently set up, with mufflers, in car, air filter, etc on the chasis dyno.
I hope this long winded response helps??
Cheers