My Accucraft K-36 is 6.75" wide (over the cab window visors) and my K-27 is 6.25 wide, (also over the visors). This is wider than the maximum width of the cylinders.
My tracks are spaced 8" center to center with a 9'4" minimum radius on the inside track on one curve on my layout. The other inside curves are 11'4' radius or more.
Clearance on curves is a bigger issue with smaller radii turns, where the pilot beam and cylinders significantly increase their overhang from track centerline, exceeding the maximum static width of the cab visors. If you are running any articulated locos/Mallets, the increase in overhang can be really significant.
My elevated table is 16" wide, so with 8" center-to-center spacing, you end up with 4" from the center line of the track to the edge of the table. It actually looks quite well balanced. With Llagas Creek turnouts, the extra-long ties that the turnout throws attach to actually extend about 3/8" beyond the edge of the 16" wide table, making them a "snag" point if you are walking or working close to the track. I had to install "guards" at those points to avoid the snag problem. Reducing the center to center distance or widening the table would eliminate this problem, of course.
Also, you can just be sure that two wide 1:20.3 D&RGW locos or big articulateds aren't running on both tracks at the same ttime!