They come with a 'T' piece so you keep the original vauxhall sender as well as the new sender.
They are the standard 2" face so I dont see why they shouldnt be able to fit in the standard cally holders.
I few people on here inc RDS has fitted after market dials to them.
Maybe they can tell you how easy.
As for when I get them I am emailing them as we speak but dont expect them for a few weeks.
I 'should' be ordering 4 each of the small ones and 2 each of the rev counters.
The shift light on the rev counter is a nuce little trick
