Nice job on the template, in that photo if you look at the tailbar where the bolt needs to extend to, it looks to have the hallmarks of being cut off with an angle grinder- note the darkened edge that's burnt the paint brown from the heat. That would figure if it was converted sometime from the smaller Chubb comb lock to make the bigger 6k75 lock fit.

So, what was there originally needs to be reinstated, so this is where any modifications or bodges that were made will have to be reversed if you want it back to the combination lock.

With an S&G 6730 fitted in position on the mount plate, you can offer up a suitably thick piece of mild steel to the tailbar, mark it to shape and size, and then cut it and file accordingly.
Note that You ideally want the point of contact down the centreline of the S&G bolt, not on the edges of it. You also want the point of contact a close fit, the tailbar plate sliding close to the bolt when it's withdrawn so maximum engagement when the bolt is thrown (extended down).

Once the plate is made, best to mark out for two fixings, drill and tap threads to fix the plate to the tailbar, say with two cap heads or hex bolts, then when everything is adjusted and all aligned up perfectly, tighten the two bolts, and remove the boltwork from the door. Finally it's tack weld the plate at a few points front and back, and then run good seam welds around the overlaps for maximum strength.

It's good practise to then clean the area up, remove any spatter (though even Chubb left it sometimes) give it all a spray over with a close match paint and you're good to go.