regarding the exhaust system, let me throw some hints at you:
1) the Series III XJ-6 (1979½-1987) will pass inspection without the front and lower cats. (assuming your State requires inspections). This was told to me by my Jaguar mechanic.
I eliminated the "Y" pipe cat (the one under the driver's side) by installing a European-version 'Y' pipe without cat. Then, the Front/Down cat was pried open, (if was coming apart anyway), cleaned out all the poison in there, (charcoal honeycomb), re-welded it, pressure tested it, and re-installed it with a new O2 sensor (any autoparts that sells Bosch will have the right O2 sensor). so it "looks" like there is a cat there for overzealous inspectors. The car passes ispections fine.
The result of all that? a faster car from zero, better gas mileage, the exhaust breathes better.
But I also took the new 'Y' pipe and the two, over-the-axle pipes and sent them to Jet-Hot Coatings where they blasted them and inmersed them in their Sterling coating inside and out. The result? a cooler, more efficient exhaust system. I can actually touch the pipes and not get burned when the car is hot after driving it. (the two rear mufflers and two rear-most resonators I found N.O.S. in Stainless Steel as the originals). If your system is the original, (unmolested), it is Stainless Steel. Mine lasted 21 years before needing service.
AND REMEMBER, no welding anything, Jaguar exhaust parts are put together with "Firegum" (not available in the USA, but you can use Permatex Exhaust Adhesive/Sealant) and only use Band Clamps, do not use "U" clamps because those deform and "crimp" the pipes when torqued. You must be able to disassemble the exhaust system in the future, for example, to remove the transmission. I found some Rolls Royce Band Clamps which are even better than the Jaguar Band Clamps, they really grab joints together well without deforming the joints.
I hope that helps, overall the exhaust is a piece of cake, I did mine myself placing the car's body on jacks and the tires over concrete blocks. You only need three donut seals.
enjoy!
Ex