I wedge a long pry bar between the frame rail and strut on the left side and pull out. Then clamp the seat tab and bend it back to parallel with the seat where it will attach.
I’m not saying it’s correct but I will push against the frame using the scissor jack method to bend just 1 bar. Also with this method, you will have to bend more or further than you need as there is a degree of return “spring” when you remove the jack.
We went from a Tillet M to a 2yd excavator bucket (Tillet 2XL) on an Eagle and pulled the weld before we were halfway there. This was pretty extreme, but keep an eye on those welds if you move it a bunch. If it gets easier as you bend it, you might have a problem
From a guy who has installed many a bathtub-sized seats on many different karts for himself, just go ahead and assume its going to crack. Have the weld re-welded and gusseted if you can (yes, experts, this changes chassis dynamics, but it doesnt hurt you as much as a broken weld every 5th session).
Of course, you can get a chassis where the seat mounts bolt on and are bent to a specific radius and angle for each seat size, but that would require a trip to Riverside…
I’m not huge but I run a size 4 or 34 seat and I have to go about 1.5" wider out of the box. Sometimes we’ll just cut it at the base and weld it back at the proper width of it’s a chassis known to crack the strut base.