Jim,
I built Arntz kit #6, 30 years ago. Like you, I got a POS Jag rear end with it. I was on a tight budget and cleaned, painted and bolted it together. That very nearly killed me! Here's what you need to do to drive the car and live to tell about it:
1. You need to reinforce the lower control arms. Mine broke at 100 mph and I got it down to 90 before it spun sideways, went up on two wheels then off the road and over a mountain. After dragging the wreck home, I used 1/4 " steel plate, cut it into triangles and welded them on. Here's what they need to look like:
2. You need to make sure you have 0 degree control arm brackets. Those are the brackets that bolt onto the differential below the brakes. There are 3 different angles. I originally put mine together with 6 degree brackets. It took me a while to figure out why I spun out so much on autocross courses.
3. The Jaguar uses a bellcrank on top of the differential to transfer force from the handbrake to the calipers. This looks like a "T". It does not work well and your car will roll down a hill when you park it if you don't modify it. The center arm goes to the handbrake lever. The other two are connected to the calipers. You need to weld a 2" steel strap onto the center arm and drill a new hole for the cable exactly twice as far from the pivot point as the original hole. Then your handbrake will actually hold your car in place.
4. After years of frustration with bad rear brakes, grinding bearings and terrible handling, I took my rear end out of the car and over to Curt Hamiliton at:
Address: 15824 Stagg St
Van Nuys, California 91406
Phone: (818) 787-9695
He installed ventilated rotors, rebuilt and widened the calipers, new brake pads, 3.73 gears, new positraction, new hub bearings, new universal joints and built a bottom plate that holds everything together.
After 7 years and over 100,000 rally miles, the hub bearings wore out and started grinding again. He rebuilt them again. Since then, I have used a modern wheel bearing grease and they are doing Ok after another 50,000 miles.
Ron Butler intended for you to use the XKE width rear end when he designed the car. You are supposed to use 12" wide by 15" Centerline wheels on the rear and 10" wide on the front. I used 10" wide Centerlines on the rear and 8.5" on the front. Ron sells plastic wheel inserts that look like original Cobra wheels and knockoff assemblies to hold them on. He may still have some. You can call him at 805-649-6000. He gave me HUGE support while I was building my car.
RS