18RG's are a rear sump engine, so to fit to a TE72 you will need a front sump and pickup from a hilux 18R-C.
Then you will need to make engine mounts.
Transmission from a 22R will fit, but might need some massaging of the tranny tunnel (can't confirm if necessary tho).
If you are looking for a cheap conversion, the 18RG is probably not for you. There are plenty of much more simple conversions for your TE72, like a 2TG, 4AGE, etc.