As far as counterweights hitting the piston at the bottom of the stroke, the typical problem is the piston pin boss, ie metal below the piston pin. This thickness varies a lot from one piston design to the next. My Ross 99496 pistons have 0.150", Wiseco K296A35 have 0.220", my Venolias have 0.180" (see attachment). Everything must be checked at assembly for clearance, but this is the math:

CWmax = Counter Weight max dia
Pthick= Piston thickness below the pin
Pin = Piston pin dia
Rod = Rod Length

CWmax = 2(Rod - Stroke/2 - Pin/2 - Pthick - Clearance)
For example:

CWmax = 2(6.575 - 4.50/2 - 0.990/2 - 0.180 - 0.60)
CWmax = 2(6.575 - 2.250 - 0.495 - 0.180 - 0.060 )
CWmax = 7.18" dia

That should work depending on the bobweight. I cut my Callies 4.250" down to 7.14" with a 2185 gram bobweight.

--------------------------
The other concern is how far the piston will be pulled out the bottom of the cylinder with 4.50" stroke and 6.575" rod.