Check out the B&G iso flanges. Adjustable flange with built in checks.
Use solder isoflanges. Looks better, one less joint to leak.
Is that a "spirovent" at the bottom?
Hard to rip the expansion tank piping without proper pics.
Thought you might need a double check rpz out there.
Last one I did, I used a TY at the bottom of the intake piping. Worried about condensation forming. Sure enough, got some exhaust sucked back in and moisture got into the pressure switch tubing. Put an adapter and some tubing into the cleanout, seems to have solved the problem.
Stickers look good.
2x2 on the manifolds look ..... ok. Paint would help.