how much horsepower can the 904 take?

There are a bunch of recipes on this site to build a solid 904 yourself for around $500 or less. So much black magic about TF transmissions. They are SIMPLE. Get the video and a book or two and learn to fish. Any parts you need can be found at places much less expensive than the popular "mopar" trans builders will charge. Northern auto, WIT, bulkpart, etc. They sell the exact same parts.

You can build your own and have a spare, or 2, for the cost of one of the builders transmissions.

If you can build an engine, you can build a TF.