Bellhousing run out checking??

[CreepinDeth]

Looking to bolt up a T56 to a 1970 Olds Rocket 350. (Going from Auto to Manual)
I'm ordering Mcleod's 8710-00 Universal Bellhousing to do it.

I saw a post by a member on an Olds forum with details on how to drill out the
pilot bearing clearance on the crankshaft , so that part I got figured out.
The part I DO NOT have figured out.....is how to measure and correct the
Bellhousing run out with a dial indicator and sized dowel pins.

I have no idea where you're supposed to mount the magnetic base to know you have dead center for starters.
Then I really don't quite "get it" how the dowels correct this.
Can any one do a "Bellhousing adjustmnents for dummies" quick lesson for me ??

This is what I am talking about. Any input is much appreciated.

The next thing I did was check the runout of the bellhousing. Its just a hair out of recommended specs but better safe than sorry I ordered some .007 off set dowel to correct it.
I tried to remove the dowels with vise grips and a little heat but all they would do is spin. I decided to make a makeshift puller by tacking a 1/2" nut on the dowels. I then took a 1/2" bolt and another nut and pull them right out by pressing against the back side of a larger socket. They came right out.