Big Block Dart Cooling

Actually, for cool-cruising, the engine wants mega-timing.I hope you have a functioning vacuum advance. I have seen good results with cruise timing well over 40 degrees. Sometimes over 50*(but not at your rpm).

How much will yours like?
Well heres a cheap test.
Hook up your Vcan to a long hose that you can suck on from the driver's seat.Tee a vacuum gauge into it where you can see it.Find a clamp that will pinch the line off and maintain the vacuum.
On your next trip, run the car up to speed and lock the pedal there.Then apply full vacuum to the can. Run it like that for several miles and record the exact vehicle speed. Then dump the vacuum, and repeat with Zero vacuum. Record the exact vehicle speed.Record the difference.And whether the no-vacuum speed was higher or lower than the full-vacuum speed. Repeat with as many different vacuum settings as you care to,to find the maximum speed obtained.
Of course terrain,slope,load,and wind, will all need to be minimized or standardized,so as not to skew the results. Here in MB, we don't have terrain and slope problems.And I have no rear seat,and the car is always empty during testing.
If the best results were obtained with the most vacuum applied,then stop, crank 10 more degrees into the dizzy and repeat the testing. Continue this way until the speed begins to fall from too much advance. There will be a plateau of around six degrees where the speed variance will not be perceived, cuz the speed-o just can't read that finely. So your cruise timing at whatever cruise speed you choose, needs to be on that plateau. Pull over. Put the light on it. Crank the rpm up to whatever Rs you were cruising at, and pull the vaccuum up to whatever achieved the plateau. Read the timing. Return to idle, and exit the vacuum.Return the timing to the pre-trip amount.
That timing number you saw, is what the engine wants at that particular rpm and mph; and no other.
Then you can repeat the tests at other rpms and mphs to see what it wants at those. Eventually you will have several data points, and then you can graph them. From the graph, you can extrapolate other points.
Then you can mod your dizzy or Vcan, or both, to try and hit the targets.
OK, so thats the hard way.
If you really wanna know, I bought a dial back timing device,adjustable from the driver's seat, and with a 15 degree capability.I mapped my mechanical advance to a graph, and same with the Vcan. I installed a permanent vacuum gauge in the cab. So from the cab I know exactly how much the timing the engine is working with at all times, just by cross-referencing the chart.I then installed the DB device in it's midrange, so that I can add about 10 degrees advance or subtract about 5.
Then instead of having to suck and clamp a hose,Plus time and re-time my dizzy, I just twiddle the dial. I also installed a 2-step return spring on my throttle arm, so I can always cruise at whatever speed I have it set to.
FWIW, my 367 at 2100, likes nearly 50 degrees. This is 16 initial, 9 in the dizzy at 2100 and 24 in the can.This is on the plateau. Here she runs cool and gets the best mileage.Occasionally I still twiddle the dial, but it usually comes back to the initial setting which I have indexed on the dial.
Yes I have O/D.
Your results,of course,will vary.