hey guys just wondering how much power the auto trans on 120y's and 1200's can take? just wondering if its better behind my E15et

Most 1200's & all 120Y's used the Jatco 3N71B trans [early 1200's & 1000's used the 3N71A]
This sambe basic 3N71B trans was also used in the larger L series powered cars along with a number of other Jap car makes & models.
The overdrive version, the 4N71B was also used behind the six cylinder Skyline models & the Comodores that used the Nissan engine, as well as being used in smaller 4 cylinder 2 litre cars like the Pintara.

The clutch packs & other internal pieces are tailored for the engine that drives it, so you could put a 1200 trans behind a six cylinder L series, after all, they were both 3N71B's but the six would cause the 1200 trans grief & the beefier trans behind a stock 1200 would suck up all the power just to drive it.

So here's your answer. The 3N71B trans that is used behind a 1200 is tailored for the 1200's output & pumping big grunt through it will apperciably shorten it's service life, but it can be rebuilt using components from the more powerful models to custom tailor a trans to your needs.

Alternately you could use a 3N71B from a bigger model that put out more power anyway.

Have a chat to a transmission expert to get all the right answers