A high flow, 180°F, Mr. Gasket thermostat (big block Chrysler size) with three added 3/16 bypass holes drilled in the nose.

Single groove pulley from a '69 302 motor, aluminum spacer to accomodate the deeper nose, and a brass plug to shut off the bypass hose - so all of the water flow has to go through the thermostat / radiator.

The spacer is 1.0" thick with a long enough nose to engage the (six blade) fan bore directly - fan mount is right on the pulley face. The smaller diameter pulley should net about 20% overdrive (relative to the original pulley). The closed off bypass may not be good for Michigan warm-ups - but my engineer's brain keeps telling me it ain't a good idea to have a 5/8 hose bypassing the radiator on a 105° day. 

I'm sure others have others must have tried this before - does it work? This is for a driver car - belts loose at the track.

I think one of the commercial 1 inch thick fan spacers will work (like from Speedway or?) - as long as the nose pilot sticks out of the C9 pulley far enough to hang the fan. I just put it on the inside instead of the front. 

PM me an email address and I can send you a PDF file of the spacer (or whatever works).

I'm thinking (and hoping) that the upper number drops due to more water flow and then the 180° low end coming up will make the idle mixture requirement a bit more stable. A guy pointed out to me that the 5/8 "short circuit" bypass hose was letting a considerable amount of water escape the trip to the radiator. 

I’ll add that modifying the bypass is a good idea providing the thermostat is modified to always let some flow past it when it is closed.  This permits the thermostat to open in a more regulated manner.  You’ve already mentioned that you have modified your thermostat with some extra bypass holes so that part is already done.  I’ve blocked the bypass in the past on other engines with success where the cooling was marginal but always made sure that there was still a way to get residual flow past or around the thermostat when it is closed.  On the Y and due to the design of the timing cover, a small amount of bypass is needed though.

For those that haven't seen the thermostat modification -here's the place where the three holes get drilled in the "Robertshaw' style of t-stat to replace the bypass.

Testing....The temperature here wasn't cool enough (below 60°F) to see the "cold weather" performance - and it won't get over a hundred here until May. In a short term idle test, the thermostat housing was running right at 170°F - still true to the 100° over ambient I found before. With the radiator cap off - and looking in there with a flashlight - things seem to be stirring a bit more than before. Seems like there could be a calibration problem with the 180° thermostat - or the three holes in it are passing  plenty of water for idle cooling at this ambient temperature with the fan and pump cranked up. 

I understand why Eddie Paskey is running a clutch fan on his outfit......but at idle it would still be moving right along as long as it is a speed type clutch and not a "thermostatic" type.

Got rid of the goofy "Caution - Fan" sticker I had on the shroud and painted the fan tips like a prop....

It makes sense to me that allowing the bypass water through there would be slowing the warm-up temperature some - but after 15 minutes in the garage at idle (even at 65°) - it should have been wide open. The Mr. Gasket "Robertshaw" style of thermostat has a 1.50 poppet and opens about .43 inches when its wide open - so there is a pretty big passage when it needs one - but at the tested temp, it would barely be cracked, if open at all. Maybe this particular t-stat is just calibrated slightly on the cool side. 

My only other explanation is that the increased pump and fan speed is doing some other deeds in this equation - like feeding cooler water into the engine and circulating it more rapidly. I am guilty of making too many changes at the same time here. Should have installed the modified thermostat - tested for a stable idle temp....then fitted the bypass plug...... and after that, fitted the pulley.

The 5/8 tube (original bypass) has about .2 in² cross-sectional area and this arrangement we are talking of using has about .11 in² total (4 x.188 holes all together).