How to get 400 hp out of 1971 340 .30 over

I don't, but I will call them tomorrow to find out. I had them shipped to my builder and have not laid my hands on them yet.
I'll update this information once I can.

I think it will be interesting to see the sizing. I'm not sure if they get that flow rating by those sizes or if the are using a straight leg booster to get that flow number.