Dipstick won't go back in

Problem SOLVED: see post 53.

I recently picked up a '70 Duster 340. I removed the dipstick to check oil level before taking it out for a drive. It came out smoothly, normally.
I cannot get it back in no matter how I try. It is in the correct orientation but seems to stop a the end of the dipstick tube before going into the block. The tube is not loose at all.
Now here is the strange part... I can insert a plastic rod (a bit smaller than tube opening) and it goes in all the way. I would think a thin dipstick would be easier to insert than this, but it's not.
I've read a lot of this same problem in my Google search, so I'm not alone, although those were all different models and brands.
Has anyone else encountered this same problem, and if so, what was the fix?
Thank you.