'Snap' oversteer on wet roads

My '73 Dart experiences a 'snap' oversteer on wet road cornering.

- Setup-front:
1" Torsionbars
Hellwig front swaybar
Poly-bushings everywhere, 'cept UCA's (Moog offset).

- Setup Rear:
7-blade leafsprings
Poly-bushings
NO swaybar

318ci / stock cam - A518 trans.
Auburn SureGrip 8-3/4" rearaxle with 3.55 gears

Two propane tanks in the trunk (1 in spare tire well and 1 under package tray).

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Even at moderate/slowish speeds I feel the backend wants to break-away / come around the moment I slightly get on the throttle.

Front swaybar has it's Hellwig swaybar struts.
Plan to replace these with Viper end links, hoping this will make the swaybar do its job better.

Also, if that doesn't help, I'm thinking I either need to increase the torsion bars diameter, or maybe take out a blade in the rear springs?

The latter is cheap, but feels like a big 'step backwards', and the car needs the support at the rear for the propane tanks.
Upping the torsionbar diameter is not cheap, but will this cure the oversteer?

What tyres and tyre pressures?

I'm guessing you have a tight limited slip differential, right?

Tyres:
225/50-17" Goodyears

Fronts are all season profile, the rears are softer winter tyres.
I would think the softer compound winter tyres would have more grip on wet roads.

Pressures are (going from memory) 2.6bar (37-38psi) at the rear and 2.8bar (40-41psi) at the front.
Mainly to reduce rolling resistance for economy reasons, but I could try and lower the rear pressures a bit more.

A few years ago I bought a used pickup rear axle with this 3.55 lim.slip unit in it. It had a cone-type Auburn SureGrip.
I used the 3rd member in my current 8-3/4" axle and used quality LSD-oil.

At first I thought I could notice some slack in the rear axle when going from Drive to Reverse or back.
And when jacked up, if I grabbed one tire, I could muscle rotate it against the lim.slip to take up the internal gear slack. I presumed the unit was on it's way out and a new one would have to be installed in time or have the old one machined.

But as plenty of drivingtime went by this noticable slack felt to have disappeared. I think the unit tightened up a bit and is working as good as ever.

The 318 doesn't have the power to break the tyres loose on dry pavement.

Rear tire pressure high and/or too much rear roll couple (spring + sway bar stiffness).

Lowering tire pressures is an easy thing to try.

There is however no swaybar on the rear axle.
Only thing I could try and tweak here are the adjustable QA1 shocks to a softer setting. But as they are not set at overly tight, I'm not sure I would like the car being softer.

First thing I thought when I read your first post was "bet he has Goodyears".

Only time I ever had a snap oversteer problem was with Goodyears.

They are especially horrible in the wet and that was with an open differential, have an LSD makes it worse.

Even worse... I gotta correct myself here;
I 'had' Goodyears All Season tyres on the car, but I changed the rears to Winter tyres and they are Minerva ECO-something tyres...

I don't have another set of 17" wheels on hand to test, so I might be looking into simply swapping the tyres front to back as a test to see if oversteer changes to understeer.

An LSD makes it real easy to bring the rear around, especially in less than optimal traction situations. You just learn to expect it and compensate by not adding power in a turn when traction is poor. Unless you want to hang the rear out.

The problem is, the sudden oversteer can happen during normal cornering if you're not careful.
Cornering on wet roads while 'always' having to be scared/prepared for a sudden oversteer/rear slide-out makes for a no fun ride.

I used to have this problem years back with my solid cammed 440 '67 Newport and its 'old' 325/50-15" tires on a SureGrip rearaxle.
Bad weather driving ment featherfooting was mandatory.

Now with the smogger 318 under the hood the Dart "shouldn't" have this issue.

Well, your polar moment of inertia is all off from stock. But you should still be able to drive it, might be a case of wrong tires, did you try dropping the pressures?

Not yet. Haven't been to my garage yet for the tyre gauge to do that.

But I'm suspecting the tires might be suspect.
Been casually looking for another set of 17" wheels so I can swap tyres (summer/winter) more easily for easy testing.
Hard to find anything 'decent' looking.