If the HW shank is as large as a Model 94 I would say it is good. I have a small shank in 40-70 SS.

In this day and age I would never put a barrel on a gun that would not support without question any factory ammunition that a shooter migth find. Fool-hardy to open oneself up to litigation. Never know where that rifle might end up and in who's hands....

Using that rule of thumb, a 45-70 at .505 would be too big for a high wall large shank action. Mayme a rule of finger would be the better choice.

Rather than guess, with a rule of thumb, just calculate it.
  
A fairly reasonable approach would be to calculate the hoop stress on the barrel, and compare that to the strength of the steel resisting the stress.  

To do so, let's use a low wall shank, and the .45-70 cartridge.
The low wall shank major diameter is .825", and it's 16 threads per inch at 60 degrees.  Tangent 30 degrees x 1/32" = .054" thread depth.  So, minor diameter = .825-(2*.054) = .717".
.45-70 is nominally .50" just ahead of the web.  So, thickness of steel remaining in barrel = (.717 - .50)/2 = .108"
The projected area of an inch of case = 1" x .50" = .50 square inches.  
At 36,000 psi, the force exerted on the two halves of a cylinder splitting down the middle = 18,000 pounds.   This is resisted by the wall of the barrel, and the area of the strip on each side at the splitting point = 1" x .108" = .108 square inches.  There are two sides with this area.
18,000 pounds divided by (.108 x 2 square inches) = 83,333 psi.

This is exceeding the yield strength of the barrel alone, at about 60,000 psi for 4140 steel.  To not blow up, the receiver would have to take part of the force, which is not a safely conservative situation to be in.

The maximum working case pressure can be backed out by the same calculations; for 60,000 psi steel, the maximum safe case pressure would be about 26000 psi for a .45-70.  Note that this has no factor of safety, but the receiver does give some factor of safety.  This would be about the max pressure that can be created by blackpowder, so it would probably be safe for that.

Doing the same calculations for a .38-55:  Case dia = .42".  
(.717 - .42)/2 = .297"
The projected area of an inch of case = 1" x .42" = .42 square inches.

At 36,000 psi, the force exerted on the two halves of a cylinder splitting down the middle = 15,120 pounds.   This is resisted by the wall of the barrel, and the area of the strip on each side at the splitting point = 1" x .297" = ..297 square inches.  There are two sides with this area.

15,120 pounds divided by (.297 x 2 square inches) = 25,455psi.  This has a factor of safety of more than 2 for 60,000 psi 4140 steel.  

I conclude from this that the small shank low wall is safe for .38-55 (.30-30) size brass, but not for .45-70 size brass.

Note that this isn't a rigorous engineering analysis, I simplified hoop stress. But, it's reasonably accurate, and is actually slightly conservative.