Last Chance:
I tested my BLI 4-4-4-4 PRR T1 Duplex on the various HO scale curves and found that 31 inches is the best compromise price wise. 34 inches was even better but had way too many track joinings every 10 degrees and paid through the nose for it. 28 it will do as a minimum.
Regardless of what track you use, there is a rule in HO, Broad curves keep big equiptment and large engines happy. You can jam the same stuff through a 22 inch train set section, but only as long as it will take for you to get disgusted with it.
Many forums are filled with posts from people who discover that uber 40-340-20 wont fit through a atlast 2.00 switch.
I was leaning toward the 790 (31in.) curves myself. That and the No. 6 switches, because I want to be able to go thru those switches at prototype speed. I scored a Proto 2000 2-10-2 with DCC for $200 at Trainfest, and I have a Mallet on order. Yeah, I need big radii for sure:)