I have to agree that you should stop, and design a plan to properly identify all the pieces so they work together.
I mistakenly figured that by now you had already done this. You were thinking correctly that your engine and gear need to work together to spin the prop at the RPMs where it would be the most productive.
I fear though, that you (and/or them) may have overlooked the intrinsic design limitations of the stock valve train. There have been volumes written here specifically about that. The biggest & foremost limitation by itself, was the WEAK and SOFT valve springs. They were only 65 lbs closed on the on the seat, which worked ok in a land barge or in a direct drive. That pressure becomes woefully inadequate for turning more towards 5000k though. If your 'engine place' simply used STOCK factory replacements, you NEED to severely limit your RPMs and your 1.73 plan will be your better bet. I thought that when you rebuilt it this last time, you did so with the intention of running the gearbox. My bad.
While your math was good figuring your ratio (ie, 5000/2800 =1.78), where you erred, was making the assumption that your engine would turn 5000. A stock valve train will not stay together, nor is an RV cam likely to pull for that long. It may spin that fast long enough to set the prop pitch, but it would not exhibit the longevity that would be expected from a fresh rebuild.
Maybe they DID sufficiently upgrade your springs, replaced the rockers, & Ts, and thoroughly checked the geometry, and I'm being overly cautious on your behalf. I believe there is no doubt that it would be well worth the time and effort to make that confirmation before committing to a gear ratio and/or prop though.
Which KB pistons did they put in it?