how tough is a A518

my ex partner and his son build mopar trans., he said they weren`t strong enough for 6-700 h.p.

How many people do we know that drive a 6-700 hp car that needs OD?
99% of those cars are straight up race cars trailered to the track.
The thing with 80's and mid 90's Mopar Transmissions is that they are easy and inexpensive to make stronger if needed.
Like I said above, they are rated for duty in trucks and vans with some pretty high tow and load ratings so in a car they should be pretty tough.
The 200R4 GM trans that people use takes a lot more upgrades to handle power than a Mopar OD does, and you don't need any expensive adapters.