ATCO D10 17" Chassis / body flex

I'm restoring a D10 ATCO 17” mower (early 60s model with Suffolk engine) - it’s a rolling chassis currently. Fresh grind on the cylinder and a new bottom blade, sharpened.

Recently I’ve been setting the cutting cylinder to b.blade engagement. I’m able to set it so precisely it’ll cut paper across the entire width of the blade without any cyl to blade contact - so a success there I’d say!

However, I noticed an unexpected issue after setup when spinning the cutting cylinder by hand. By biasing the the mower’s weight (minus engine at this stage!) to left / right on the front roller by sitting it on a wooden block - there is an audible change in blade engagement. This audible change in engagement indicates there’s some flex / twist in the chassis.

I’ve checked and everything is tight & assembled correctly. Is this chassis flex a normal & known issue? Am I being too fussy?  Any help or suggestions welcome before I look at strengthening the rear cross member which is where I suspect the flex is likely to be coming from (circled in parts diagram below).