I think the best way to attack it is on your HX, cut the turret off 10mm or so above the quarter line from doors to tailgate. This will allow you to dress the join with a grinder and flapdisks so you are left with a clean quarter top with no holes.

Then do the opposite on the WB, ie cut under the quarter join. Then you can drill the spot welds out from below and remove the remainder of the quarters. This will leave you with a neat turret with spot weld holes.

Once you've sorted how you are going to do the windscreen pillar joint, you can then sit the turret on and weld it down with a mig using the spotweld holes as points to weld. I'd be using a non-porous weld through primer on both surfaces though before bringing them together. Not sure what sealant was used on the joint but talk to a few beaters about what is around today, some of the sikaflex stuff used for panel fixing might be a good idea.

I've seen windscreen pillar joints done a few times where people have welded a Statesman firewall and floor into a tonnner cab (to fix rust). You will need the advice of a beater first, but I have seen them done where the pillars were separated in between the holes that hold the stainless trims on (for exact measurement purposes). What they did was unpick all the spotwelds in between these holes (there are heaps of them and it looks like swiss cheese once done!), and cutting the lower and upper a distance apart so that they join up staggered with at least 6" of overlap. Then you have 2 x butt joints plus the length of the overlap to weld. At some point in these pillars there is also in inner SHS like tube. Maybe find a rusty HQ-WB somewhere and cut a pillar out so you can investigate where the best place to join is?

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As Mook says I do remember a difference there, maybe look at them both close first and see where they differ.