Need to rebuild my 727. STRONG

So a few days ago I was just cruising and the trans starting slipping badly in top gear. I suppose the previous thrashings didn't help.
I am going to have a mate rebuild it.

Now my question is what parts should we put in there to strengthen it up.
The next engine build will have around 700hp so it needs to be able to handle that and a bit more.

I have seen the low band apply thingo so that is a must.

Any help will be much appreciated

Cheers
EXcappa