There are no Capris pushing this kind of power so we don't have folks here reporting their findings firsthand.
The info I have gathered on transmission reliability comes from watching various Mazda forums over the years.
The Bore/Stroke ratio of the Capri is relatively square, with a tad more HP vs Tq, they are roughly even.
Like I said above, the weakest spot of the transmission is the diff, if you correct that then you should be good
There's lots of good info on the Clubprotege forum, I suggest having a poke around there. There were guys going thru extreme measures to get the g-series beefed up with varying levels of success but also $$$$ investment. Billet bearing cup/plate in a modified case, external framework around the case to prevent flex, custom gearsets, etc