Big Block Dart Cooling

You cannot damage the Vcan thru the nipple.
I would Skip the water wetter. It is very expensive, and I doubt you need more than straight water.
For the amount of time that the water is gonna be in the system,
personally,I would skip the distilled water too. If you can drink it, I would dump it in there. It'll be out in a few weeks; when you get the 7-blade all-steel oem-type giant fan on there, with the thermostatic clutch.lol Then you can go back to an antifreeze mix.

You say you have power-timing of 36 to 38. But you don't say when it's all in by. If it's in early and the Vcan adds it's load early, Then it could all be in too early, and surging is the result. So then you disconnect the Vcan, and then it's all retarded for every circumstance except WOT.Retarded timing consumes more gas than necessary, and that makes more heat than necessary.
If it surges as you describe, you have too much low speed timing. In your case, I would probably back off the initial and put the Vcan back on to the sparkport. Unless you currently have it hooked direct to manifold vacuum, then I would swap it to the sparkport first, and see how that goes.
You also don't mention your cam specs, or your compression ratio, or your cylinder pressure;and that is important in determining idle timing.
I'll go out on a limb and and say ALL your timings are just plain too early, and some are too much.
I see you are a one-change-atta-time kindof guy.
And if I were you; I would map out the timings and see where I'm at.Then I would start making changes. One atta time. Well no, I would skip ahead a bit first. I would set the initial timing to 16/18 at idle speed. I would ensure the T-port sync is correct.I would limit the power-timing to 36* for now. I would slow it down so that it is not-all-in until around 3600.I would hook the Vcan back up...to the sparkport. I would measure what it is capable of. I would unwind it's adjustment to allow it to work as soon and quickly as possible.Now I would map the timings out.From the maps I could see how much timing the chambers are seeing for every driving circumstance. Next, I would roadtest it to see where and when changes need to be made.That's my plan.
I don't know if you are one of those guys who subscribes to the more timing the better rule, but if you are, I gotta tell you, those days are gone. With pump gas, a street car needs pretty accurate timings. That engine is being asked to run and perform under a myriad of different conditions and circumstances, and so it needs a careful choreography of the timings.To that end, it is advisable to sneak up on all the timings. Save perhaps the WOT timing from somewhere in the mid 3000 range to shift rpm, cuz that is more or less written in stone.
More timing is not always better anymore, and in fact, 3 or 4 degrees less than optimum is very often better than just one degree too much. So my very best advice is to start conservative, and sneak up on the optimum. The graphing of your curves will help make that as painless a process as possible, if you start with a conservative tune.
IMHO,The low-speed timings for a street car with a conservative TC,are far, far, more important than wringing the last 10 or 20 hp out of the engine at peak power.And if you change the stat midstream, you get to start all over.

So the order of operations is;
1) Bring the cooling system up to speed,whatever it takes;make sure it is up to the task.
2) set the minimum engine temp with the stat.on the conservative side.
3)ballpark the timing conservatively, for the fuel you will be using.
4) ballpark the AFRs.
5)Sneak in some timing and adjust the AFRs as may be required.
6) repeat 5) as often as necessary;trying to hit as many targets as possible, making as few sacrifices as may be necessary.See note one below.
7) try a lower grade fuel, and go back to 3) and start over.
8) increase the minimum engine temp and go back to 1), and start over.

note 1;
For a streeter,the targets are; a)low-speed driveability from idle to about 3200rpm, b)idle quality, c)freedom from detonation as throttle is increased, d)fuel consumption, e)detonation-free power, f)and should be in the order listed.
The sacrifices may be:idle quality,fuel consumption,and/or, cost of fuel.I would sacrifice ultimate power for driveability.
Detonation and overheat is/are to avoided at all cost.

All the best to you