This pic shows the rusted wear witness line (rub line) of where the A frame bottom support plate touches the jack "outer" tube. The rust line is about ~ 1 1/2' to 2" above the bottom of the outer tube.
Point being, the hole in the A frame support plate has to be equal to or slightly larger then the outside diameter of the outer tube, not the inner tube.
Your jack foot by this pick, comes out to be 1 7/8" OD if your tape measure tip if not out of calibration.
Not sure where you are at on this right now, but this might help to sort out what the A frame support plate hole really is.
Since you have a tape measure, there is a more accurate way to measure then to use the silver angle tip to declare your starting zero point. The sliver tip slides back and forth to be able to do inside and outside measurements with the silver clip. The sliding action compensates for the thickness of the silver clip. Sometimes that sliding tip gets made wrong, worn/bent wrong or used wrong. I can explain more on the silver tip etc. on why it can more more inaccurate for close measurements if you want, just ask, but try this instead.
If you want to get a more accurate measurement, you do not use the silver tip. We use a method what we call, set your zero mark on a whole number in place of the silver tip. It can be any whole number, but in many cases you just pick the 1" line mark.
Using 2 hands helps to steady the tape measure blade, line up by eye the side you want as a starting point as exact as you can with the number 1" line. Try and get the 1" mark dead on the edge you are starting on.
Then position the tape measure blade "edge" you used as the 0" mark as exact as you can, right across the center of the item you want to measure. In this case the A frame lower jack tube hole right down the center of the hole. Then look at the dimension on how long it is, and read the number. Double check your 0" mark is still right on the 1" line. Then subtract 1" from the total distance. This method still has some small error in it as you are eye balling the measure points, but it is more accurate then using the sliver tip if you do not know the tape is still in calibration.
You may have to get on the ground and look up at the hole in the lower A frame support plate to measure it as getting in on the top is too tight and the ball coupler is in the way.
Another way to check what the old jack outer tube is, is to measure the outside circumference. You know that old jack tube fit in the hole and that is what you want the new jack to be. I did the math for you, but circumference uses a formula: Pi(D) = Circumference. Pi = 3.1416, D = the OD of the part you are measuring.
For a OD of 1 7/8" we use 3.1416(1.875") = 5.890" and that decimal number is 1/64th larger then 5 7/8" on the tape measure.
For a OD of 1 3/4" we use 3.1416(1.75") = 5.497" and that decimal number is very tiny bit (0.003") less then 5 1/2" on the tape measure.
You can see that there is good size difference between a 1 7/8" OD and a 1 3/4" OD to the tune of a little more then 3/8" and that is really easy to find on a tape measure.
Take a tape measure, (a smaller width blade works better then a wide one) and wrap the tape around the tube OD. Use or 1" or 2" as your zero mark and read the circumference distance less the 1" or 2" offset for zero.
Here is a basic show and tell on doing OD measuring. https://www.wikihow.com/Measure-Pipe-Size
You can also use a piece of paper to wrap around the OD of the outer jack tube. Mark a starting line on the sheet of paper, wrap it tight around the OD back to the the starting line. Mark the paper where the starting line aligns with the full wrap. (You can use the edge of the paper as your starting line if wanted) Lay the paper flat, then use the tape measure and start on 1" mark and measure the length between the 2 marks and that distance laying flat is the circumference of the old jack outer tube.
The Bal jack had a 1 7/8" OD outer tube, if your old Atwood also had a 1 7/8" OD tube, then all is good with the new Bal jack, just get the one with the right jack post travel.
If the hole is still too small, the I can explain hole to grind or file it out larger to fit.
Let us know how it goes or you need more help.