I did hardened seats when building my DZ recently. I've beat myself up over that scenario many times over the last 20-some odd years. To do or not to do. Anymore now I just do it on every build. It's not overly expensive and really there is no reason not to do it. Piece of mind for me since I plan on running the car in PS, plus it's daily driven so it will get used a bit. Zimmerman Racing did mine and he uses a specific hardened seat for the 1.60 valve that he prefers. No trouble at all with water jackets. Good to use a high end shop with experience, pay a little more but better end result. I also like the hardened seat idea to avoid the possibility, or at least reduce the risk of sinking valves which affects your lash adjustments if not checked periodically, but more importantly, it affects spring pressures, something I'm pretty picky about on a high strung engine.
Even with all that said, I built my Formula engine 20 years ago to go PS racing, and it's been run pretty hard over the years along with long road trips (several hundred miles at a time). I didn't do hardened seats on this build, and it's run pump gas since day one. I just recently pulled it apart for a refresh/inspection and the seats/valves looked beautiful with virtually no wear, valves sat up nice and tall on the seats like it was just put together yesterday.
I do run a little 2 cycle engine oil mixed in with my pump gas to add some lubricity in the fuel. I tend to think that helps the situation.
To the OP, if it were me and needing a set of 186's, I'd look at any of the 1.94 sets and not think twice about it, Port size and combustion size are identical, and will open up your ability to find a good date. Because as far as I'm concerned, any set I'd use would go straight to the machine shop anyway for a new set of valves, guides, seats etc....and when doing all that it's nothing for the shop to go ahead and upgrade to a 2.02/1.60 setup. At that point you have yourself a set of Z/28 186's.
