In my (very) limited experience, i think a lot comes down to the skill of the person dipping.
I would imagine you are pretty safe with an 8mm plate, that's, effectively, reinforced, but heat does do odd things. My money would go on you being fine, but someone with more experience should be able to give you better advice.
i think you might be better with this
A side note -My experience of the dudes at Crediton is that they can bend anything
He seems happy with the design, it’s more the galvanising process he’s asking about
I would have some concern about the plate oilcanning even though it is heavy.
Is the plate there just to add weight? It has quite a large surface area to weight ratio, so its galvanising cost will be relatively high. If you could replace it with solid (square or rectangular) material of the same weight, it will have a smaller surface area and be cheaper to galvanise. This might also allow you to move more weight out to the perimeter for even better stability.
The box sections won't warp and they'll help the plate from warping much if at all.My experience of the dudes at Crediton is that they can bend anything
I can't decide if the plate is going to try to distort the angle frame, or if the frame will prevent the plate from warping in the first place.
if you want to build it like a pallet then build it like a pallet
the middle will bow over time and any rocking of the mast will create fractures due to it not being supported
all it needs is a cross member inside of it with cut outs for the forks to go through from any angle for lifting
You could always send the plate separately and bolt it to the frame, far less chance of it distorting then.