What Part number is best to Use with a 4.500 Bore
On their Web sight they list 5 different thickness, Do the thicker ones seal better?

I don't know about sealing better , but the thicker gaskets are used to set quench and determine static compression ratio.
edited to add.........I should say different thicknesses to set and determine.

I would use permatex copper spray on the cometic gaskets

i just pulled my invoice from them, H1637SP3040its listed as a 4.530 bore. this is what i used with the 4.500 bore as per there recommdation.

Customs is what I have always done. Wast no more money...
I added .060 to my Hemi for a gasket bore or 4.56, adjusted the thickness to whatever.. Like .035 or so...
They are tough. First on the Wedge I used the copper spray, didnt like how they torqued, which was spongy, and lost torque a bunch.
On the hemi I installed them dry and just added a bottle or Bars Stop Leak.
Never a problem

Put the copper spray or a little copper rtv around water ports and do a retorque after some hot/cold cycles I dont care what they say (no retorque required)