I have many bikes that I randonneur on and also one that is a 650B conversion. I would strongly recommend getting a higher quality frame that fits OR buying someone else's mistake.
I got an unbelievable deal on an older custom made frame with True Temper OS Platinum tubes. I built up a set of 650B wheels with good quality parts and put on Rene Hearse Extra legere tires. I had pretty much all of the other parts but if I had to purchase them, I would into this bike for a lot of money. The bottomline message from me.....it won't be cheap and starting with an undersized frame of mediocre quality is a mistake. Sorry to be so blunt.
I also have another randonneuring bike that runs 700 x 35 tires or sometimes 32 mm.
They both are more comfortable than a full on racing bike using 25 or 28 mm tires but the racing bike also finishes the randonneuring brevet much quicker. There is no contest. If your objective is to have a comfy bike for rides under 100 miles, you are on the right path but expect it not to be so inexpensive. Do yourself a favor.....start with a different frame.