are you going to make any more of the old design? The new design requires force against the head. Heat cycling here over time will allow it to bend and leak. Not right away, but in time. If you use this type you need direct pressure at the head. Fender washer would help. But I believe the best bracket for direct pressure AND room for the fitting is the hold down for a small block Dodge distributer. The early type uses the unit to press side ways inside the whole. The new design requires force at the head. That far away it'll fail in time, may not work at all with some heads that are rougher.
I like being larger diameter though, even with the O ring inside having the thing wider would help. The other option for force at the O ring is using the lower support hole as well. Too much work though, adding the old O ring to the new wider plate would be faster.
as you can see to get force on the nose they go back and have rear support. Making it longer on the cam side and adding a 1/16" bump on the head side would also put force on the O ring. Like a dist hold down a bit, the gap under the bolt allows force on the O ring.
I think the face seal version gets the job done (see below), but, this is an experimental design thats still evolving, so if you have a strong feeling about the bore seal version, I'll make one just for you, to your specification. I can make it bore seal with a hole for the lower bolt as well as the upper one. Thing is, I have to buy a $75 keyway cutter to do it right, so i'd have to ask you to buy the kit for $75 shipped. Just buy it directly from me so I don't have to pay Ebay's 9.6% highway robbery.
So heres my shpiel on the current design:
I know the cantilever face seal looks like it might be subject to bending or warping or some other kind of failure, but if you look at the data, I'm not asking much from 6061-T6. Stress fatigue is 15,000 pounds/sq in for 100 million cycles to 40% tensile strength, at which point the oring would still have 10% compression on it. At 300F for 10000 hours, 6061 loses 25% tensile strength, so even less of an effect. And since its a chunk of aluminum bolted to another chunk of aluminum with coolant flowing through it, I don't think enough of a gradient could form to cause warping.
It does, however, matter that the bolt in question be secure and put pressure on the adapter plate evenly. And it does matter that the design limit deflection from the static load of the oring, which is almost 100lbs, and resist bending under normal forces (pulling on the hose, having a tool dropped on it, who knows). So the design that ships right now is improved from the pictured version as follows: its 0.1" thicker which increases stiffness a factor of 9, and its wider at the bolt end. It also comes with a flange head bolt and thick washer.
I like the bore seal design, but it not without its flaws. I've yet to not scratch the bore surface pulling out freeze plugs, and once you've scratched it, you're already standing there with a kit that wont work anymore, a hole you cant really plug, and the coolant drained. At least with the face seal you know if your head surface is good or not before you buy the kit.
By the way, I've just received my shipment of fittings, and I get hose, orings, and hardware tomorrow, so all kits ship within 2 days! (not yours though Pope, I have to write new toolpaths for it and the cutter has to come in, so you'd be waiting a couple weeks hehe)