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'm looking at Lewmar's 2006/7 dead tree material catalogue. The 29901624 (60mm fiddle, cam/becket/ratchet) is shown with a 400 kg SWL (882 lbs) and a 800 kg breaking load (1764 lbs).
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.
There's a thought.
(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.
Yeah, the 3:1/6:1 is not suitable for the amount of sail we have with mid-boom sheeting.
I'm thinking of the KISS principle, here. I'm thinking maybe just going 6:1 and being done with it.
What do you think of this combo (discovered by The Admiral, I would note):
Harken 2141 57 mm Carbo Triple Ratchet w/Cam, Becket
Harken 2604 BLK-57MM CARBO TRPL
The limits are imposed by the traveler-mounted end, the 2141, and are based on the cam cleat. We'd have an SWL good to 27 kts and a breaking load good to 43 kts, based on Harken's mainsheet loading calculator.