If you're welding on the frame, you may still have problems with a 90 amp 110v welder since the frame dissipates the heat so quickly. I upgraded my Hobart 110v 135 amp welder to a 220v 180 amp Miller welder because the Hobart wasn't able to weld big pieces of 3/16" plate steel, though it could weld small pieces of 3/16" ok. So your 90 amp may be pretty well near its limits when welding on a large surface area of 1/8".
And the manufacturer was exaggerating when it said it could weld 1/4", it'd be a stretch to weld even 3/16". As said above, my old 135 amp Hobart struggled with big pieces of 3/16", let alone 1/4".