1973 340 issues.

About modifying the compression tester. Probably yes.
If its the type that screws into the spark plug hole.
On mine, the hose attaches to the gage with a air hose type fitting. But I couldn't use it directly. It's at the shop, but IIRC, the check valve would have blocked the air. So, I removed the spark plug adapter end and attached it to a airhose coupler that matches my hose fittings.

I was doing this to change a valve spring, but I think everything applies to what you are going to do. I did the busted one first. When I saw some other springs were pitted, I bought more and a brass 14mm spark plug to npt adapter since I found they are fairly cheap (here) - so that's another option.

Edit: Be prepared - The engine might turn when the cylinder is pressurized -