I have an 1845C with a Simms cab and the Zero Start heater box. The compression fittings on the back started to leak, so I pulled it out and found the ferrules were wore into the tubes croaked and it couldn't be saved. I got a replacement and cut the new tubes to accept the 90 deg compression fittings. I didn't get new fittings, just new ferrels. I soon found that the fittings wouldn't seal. I got 1 new fitting, as parts stores around me don't stock 5/8" compression 90ies. The replacement core had one of the tubes twist and crack where it attaches to the core. I broke down and bought a 2nd new core. Are there any option for 90 deg fittings that push on?
Would a molded heater hose 90° elbow do what you want ?
Where do you get those?
I obtained that image of a silicone elbow on EBay. I simply did a Google search for 5/8 inch molded heater hose 90 elbow. Here is a LINK. Most auto parts store have them available in different lengths.