First of all, don't make the mistake of another member here who's machinist flooded the jackets with epoxy ... the dang thing ran so hot he had to replace it!
Plus, I'm not so sure primering is where it is at ... I've never seen it instructed to use primer with DuPont Liquid Steel. Primer is relatively soft.
Metal impregnated two part epoxy grabs hold of roughed up raw metal extremely well ... and probably much better than having a primer involved.
I do like the tecqnique some have used of beveling the deck edge of the jackets so as to create a larger plug 'head' that will help prevent the head gasket from pushing in the plug on torque down.
I also recommend using the biodegradable type of packing styro packing pellets to stuff in the jacket areas under the plugs. You can determine this type of pellet by dipping them in water ... they disolve almost instantly.
This prevents the heavy liquid epoxy from filling/coating the internal head jacket areas which will prevent good hot iron to coolant heat exchange.
As to a 'special gasket' ... there are no corresponding openings in a 4.2 or 4.0 block that would line up with the twelve outside triangular water jacket openings in the HO head. There are no corresponding holes in a 4.0 gasket either.
The reason for even messing with pluging these is due to the close proxmity of these openings to the edge of the block's deck ... some much more than others.
After trial fitting the head to the prospective block, many have opted to do nothing to the head due to their having 1/8th inch or more gasket seat area between the jackets and the block's deck edge.
As to welding, both Clifford and Hesco (and probably Advanced) weld the jackets to eliminate any of this type of problem of arising in their products.
Welding is the best way, no argument.
I know from experince that that Hesco uses a filler rod that can't be color distinguished from the surrounding jacket metal after re-decking to clean up the welds which leads me to believe they do not use a copper alloy rod.
Bottom line ... it's still the best bang for the buck of any Jeep I-6 mod.
Especially when combined with fuel injection.
The major web articles on this swap are listed here ...