The 20mms are standard USN mounts but the 37mm mount was unique to the PTs. It was based on the same gun fitted to the P-39 Airacobra fighter.
For a quick answer and probably much better shots, go to Al Ross athttp://www.coastalforcesplans.com/
or at any of the PT forums.
Al has written much of the technical and historical work on PTs in the last 40+ years. He & John Lambert have done hundreds of detail drawings of virtually every PT & Coastal Forces boat and fitting! After your kit came out, he assembled a packet of detail drawings of fittings and weapons and details specifically for the Italeri kit.
Good luck--it's a great kit.