Given that you’d have 88’c thermostats, the minimum temperature of the coolant after it exits the block is 88’c, as the thermostats would otherwise close and recirculate the water. The operating range is ~12’c, so that gives you a range of 88-100’c on a running engine. All of the exposed metalwork will be dissipating heat, so I’d expect those numbers to be a cap. Post shutdown, heatsoak can probably add 10’c to that, although that will then dissipate after time. I base that on datalogs from my etype, which has more space under the hood than the xjs has, but the heatsoak part of that you can view from previous posts about solving v12 heatsoak.
Excess over that would result in venting to the expansion tank, so that’d be an indicator of temperatures reached, given your cap rating.
(Additional, but not quite relevant info, is that a non working cylinder has an EGT of ~160’c if the other 11 are running.)
My 88 had two vacuum temperature sensors screwed into the water rails. A side was for emissions. B side was for charcoal canister. I pulled both systems and reused the holes for my A and B bank aftermarket gauges.
If you can’t do that, you’ll have to remove water rails and tap into aluminum ends. Plenty of room.