I also have been down this road Colesy, I'm surprised you got the Mikuni float valve to seat with 4 psi ? I had no luck @ 3 psi (max)
Have a look at Sumner's "formula" pages to work out your fuel supply needs (HP v's fuel required), then measure the flow out of the bowl cap orifice (after removing it) & see how the flow relates to your requirements 1st.
I had to go to a 70 gph low (5-8) psi pump to get the flow I needed , then find a way to control it,,
Here's my solution,,,,(the HSR was way too big for the 250)
It's an old Holley float bowl tapped into the Mikuni bowl(floats & fuel valve removed from Mikuni), the setup safely handles up to 8 psi, and flows 2 liters/ minute (methanol)