Where to get a gear drive conversion kit?

[tireshark]

Russ Johnson, Southside Equipment. HP5535UR.

Instructions on how to swap it in this thread:

Upgrading my GX390 to 5.5 GPM... question

Have a direct drive 4GPM pump on it, going to a gear driven 5.5GPM. I’ve had this pump suggested to me: (Not buying from that site, just linking it for the pump specs.) It says 3500 PSI, but anytime someone talks about a 5.5 GPM, they usually say 2500 PSI. So what gives? Will the pump put out 3500 PSI with my GX390, or will it be down around 2500?