My first guess would also be a bad or leaky compression gage. Most have a check valve built in that can hang or screw up.
After you've tried the easy stuff and if it doesn't work out. Take off the valve cover and turn the motor over by hand until the number one piston is all the way up and you can wiggle both of the number one rockers, (both exhaust and intake) the valves should be fully seated and with a little play in the rockers. Check the timing marks, they should be close to Zero.
Remember you have four strokes and both the valves (on number one) should be fully closed on the compression stroke, with the piston at the top and the timing marks should align. You may be a couple of teeth off on the timing chain. It isn't common for the I-6 to jump timing, it may be impossible. But the chain can be installed wrong and there are two different tooth counts available (maybe more), one change in the tooth count was right around 93 or so. Look and see if there is any fresh looking silicon squeezed out on the timing chain cover gasket.
You can get the top of the number one compression stroke almost exact, by using a piece of welding (or coat hanger) rod with a slight bend in the middle to feel when the piston is all the way up and starts back down again. It may take a few tries, with the last movement of the engine while you are turning it by hand to the right, after backing up a quarter of a revolution. The object is to back up far enough to turn to the right again (as you are looking at the front fo the motor) and take up the slack in the timing chain. You have less chance of backing out the harmonic balancer bolt, if you take all the plugs out.
Timing chain jump is fairly common in the old Dodge LA motors, it would show very low compression on all cylinders when this happened. I'd be careful, on some motors it's possible for the valves to contact the top of the piston if the timing chain is very far off. It may or may not be a factor with the I-6 motor.