So what we're experiencing here, or likely to experience here, isn't necessarily what we'll experience elsewhere. Indeed: Even just going across the state, to Lake Michigan, will be quite different. (I understand Lake Michigan can become quite harsh, quite quickly, quite unexpectedly.)
Now you are thinking more "globally"
I mutinied. I called up and canceled the order late yesterday afternoon. (Made it just in time, too.)
60mm, actually. It's not so much the 60mm, but the SWL and breaking strength.
Wait... You quoted SWL at 19kts; breaking at 27. I put the following numbers into the Harken Mainsheet Calculator
E = 11.8
P = 33.5
V = 27
X = 4
With those numbers I get a mainsheet load of 1772 lbs. Since you stated the blocks you selected will fail at 27kts; I assumed that you were talking about the 50mm blocks. The 60mm blocks have a SWL of 1760 lbs; not failure load. I think the confusion lies where they show SWL in both KG and LBS on their spec sheet, but they don't quote failure loads. Since the 60mm specs don't include failure load I can't tell you at what windspeed you would expect a failure at; but the Lewmar Selection Guide
shows the 60mm blocks as being suitable for mid-boom use on boats up to 34 feet; with 72mm blocks also suggested. However, the header states "for hand held and winch control" which suggests that a mid-boom setup should have a dedicated winch (or extra leverage that their 6:1 blocks are not well suited for).
Now I just need to decide between the Lewmar 80mm fiddles, the Harken 4:1 solution (57mm dual block, 76mm fiddle), the Harken 6:1 solution, or maybe the Harken 4:1/8:1 self-contained system. (I'd really like to go w/Lewmar, but they don't appear to do anything with a ratchet, other than fiddle blocks.) Decisions, decisions. The lower I go, the faster I can move the boom and the less mainsheet I'll have on the cockpit sole, but the more strength it'll take to use. The higher I go, the slower the boom moves, the more sheet on the cockpit sole, but the more likely The Admiral, and perhaps I, will be able to deal with it. I think a tour of the club's slips is in order, as Sailormann suggested.
I'm thinking your biggest consideration among these choices is the amount of leverage that will be needed. I don't think the Admiral wants to develop ape-arms while trimming the main; and as for the additional sheet you could get a sheet bag to hold the excess when you are sailing upwind.
If you want to minimize the amount of sheet you need, you would want a 4:1/16:1 setup but it looks to require two mounting points on the boom for single blocks. (You should check that mount on the boom BTW to be sure it is properly sized, since it was retrofitted in).
The 4:1/8:1 that Harken sells is essentially an 8:1 system that has double bitter ends. It works by pulling on both lines to get a 4:1 sheeting speed/force, but you will need 8:1 line length to make the sheet long enough. They also make a 3:1/6:1 but it might be too small for your mainsail. If it were me; I would go with the 4:1/8:1 or a 4:1/16:1 with separate sheets.