Mike,
Shock specs depend on which Jag rear you use, as the lengths of the lower arms (wishbones) are different.
As far as I know there are three stock Jag lower arm lengths, from shortest to longest: XKE, 3.8S (and related), XJ6/XJS. Track width varies accordingly.
The shocks you need will depend on which arms you use, or if you are changing the lengths of yours (i.e. shortening the XJ6 arms).
I use the middle length, for the 3.8S sedan length wishbones, and the four shocks are from Aldan: PAS657.
I agree there is a "correct" length for each, as mine were too short to begin with and made for some interesting pucker moments on the track; as the car rolled a bit during a turn, the inside wheel would lift - picked up by the shock which was fully extended - and cause the car to skip sideways. Lengthening the rear shocks helped immensely!
Studying the ERA site (their 427 Jag rear specs specifically) leads you to this shock they use:
Rear Jaguar damper, adjustable
Recognize your shock mounts may be different either from theirs or my Aldans...
Happy hunting,
Tom