I have been trying to install a dipstick tube (chrome) in my 400 and it is giving me grief. When I first discovered that it wasn't going to go in the hole without a fight I looked up the topic here and found a previous thread in the subject. It was said that the chrome tubes need to be sanded in order to remove some of the chrome to get them to go in the hole. So, I sanded it and tried getting it in the hole probably a half dozen times to no avail. The previous thread also suggested freezing it first. I tried this, again to no avail. So I went back to sanding and eventually I got it to where it started going in the hole. So I put an open end wrench over the flange at the bottom of the tube and tried tapping it in. It started to go in but only went in so far and now won't go any further. There is about 1/2" of tube below the flange left but the hole in the mounting bracket is about 3/4" off from lining up with the motor mount boss. So I don't know what the deal is. I don't know if I can get it back out now to try and sand it some more but the fact that it doesn't look like the bracket is going to line up with the boss has me more concerned. Suggestions?

I had the same problem with mine. Ended up damaging the tube by hammering on the wrench too much, trying to get the tab to line up with the mount ear, and when I tried to remove it, snapped the tube off. (You can read my adventure in removing the stump in the Q&A forum. That was really fun)

You will need to sand the tube down enough so it fits in the block bore snugly but not super-tight, then coat it with a thin layer of your oil-resistant sealer of choice...

I assume you have already checked the block for paint and other debris in the hole?
Also - make sure the hole is drilled deep enough for the tube you are trying to insert. If not, you'll also need to shorten the snout of the tube.

Edited to add: It is possible that the part has been made incorrectly, too. I have read complaints on some brands that the tab is in the wrong place, or that the length is wrong, or both...

Some of the tubes, especially the chrome ones, need the tips ground down on a grinder to start in the block holes.
I use a 1/4 x1. 0 long inch bolt stuck into the top of the tubes to drive it into the blocks, I do straighten the top part first so it won't bend while driving it in with a small hammer Not a 5lb sledge hammer Once it is in as far as I can safely get it I rebend the tube to fit the block engine mount holes
On your deal you may need to remove that tube and make it shrink back down to original O.D. size to get it in all the way to the stopping rib

Yup, I've had the wrong tube length and the wrong tab location on aftermarket tubes.
Gotta sand the heck out of them, or grind 'em, as far as diameter goes.
If they're a loose fit, no worries, seal with a non-hardening sealant and you'll be fine as long as the tab is mounted.
I don't trust the dipsticks either, had one that was almost 2" too long. Not a difficult fix, though.
If it's not going in all the way, it'll never be easier to remove and solve the problem than it is now.
Then again, maybe "easy" isn't the best term...

I used a small box end wrench over the MP chrome tube into my 340 block. I hammered away at it with the wrench against the bulge. It worked OK, but there must be a better way.