I assumed the FDC was on the same level as the bridge. You never mentioned that and it is key info.. Now I get why you mentioned 600 lb flanges.
Just a side bar if you want to learn something else; from my days as a pipe stress engineer; I believe you will find that 300 lb flanges are good for up to about 400+ psig or so at water temps.. The 300 designation is roughly the pressure rating when the metal is at a temp of 300 psig saturated steam. Eg 150 lb flange good for ~150 psig at a temp of 366 F. At room temps, the allowable design pressure goes up. It is easy to google up the actual allowables.
But to stay on subject, the solution here is to change the FDC location to be on the same level as the bridge.
There is no brand of flexible coupling required. Every company that makes grooved fittings makes a flexible coupling. Victaulic is one example.
BTW If you are a "hatched plan mechanical engineer" as I call them, and really want to learn about fire protection buy the SFPE Study Guide for the Fire Protection Professional Engineering Exam. It would help you fill in knowledge gaps regarding standpipes, sprinklers, passive features, life safety plans, fire dynamics. I have a strong hunch you don't know everything you should know about sprinklers, water supply, or pumps to keep yourself out of trouble in the work you already do regularly. I am a reformed mechanical engineer myself (BSME) and it takes one to know one.
Best of luck on your project. I'll bow out of this thread now.
Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.