you cant turn a unibody into a ladder chassis.no matter what sort of hole you put in the floor, the jeep floor from an xj will still twist and flex.a reasonable radius on each corner is all you need and then cut the straights with a grinder. no need to go over the top but keep flipping the carpet to see how much of an effect the froading does in regards to the new hole. I would paint a 10mm edge all around the hole in white as this will show any crack quicker than a dark colour.If a crack does appear then drill a 2mm hole at the end and that should be enough for a stress relief, this obviously will be dependant upon the type of of froading and flex is being seen
Cut mine to access the pump qhen i had the lpg conversion done and no problems at all. Managed to change the pump in 10 mins when it went out from lifting the carpet to laying it down again:-)
How about two circles of 1/4" steel big hole of 8" in the Middle.One piece above and one below the floor drill and bolt at 12, 3,6,9 o'clock.Or just weld together for a nice Fat 1/2" sandwich type thing. Can't see that splitting the floor???