1977 904 horsepower limits?

Hello guys.

In bone stock, 60k mile trim....what would be the boom point on a stock 1977 904?

I know it may be a silly question, but most transmissions seem to have a "wives tale" type hp limit.

Any input on flywheel hp?
350? 400? 450?

Would a manual valve body prolong life at all by eliminating the slips between shifts?

Thanks all for the opinions.