From: Crawley, west sussex
Autometer make a trans temp guage and fitting kit with instructions, not cheap but considered the best
Now I think you have a choice of where to install the sensor, You can mount it in the oil pan/sump of the transmission or "T" into the return line from the cooler.
To "T" into the line I would personally do it into the steel return line using compression fittings, Not sure if these are supplied with the fitting kit or not, But if you need one then any hydrualic specialist should be able to supply the correct size, all you need is to accurately measure the outside diameter of the steel tube using a caliper or micrometer and the hydrualics man should be able to help you out.
It Is a shame that Darren is not about any more as I am sure he could help with advice and parts to do this, alternatively ask on www.astrosafarivans.com
Former owner of "Lone Star" 93 Astro Tintop.
Now Van Less, but still love the hobby and the people in it.
CLEVERLY DISGUISED AS A RESPONSIBLE ADULT