It really sounds like the issues you have with your Blackhawk are ones that you could easily correct with firelapping. In fact the issues seem so common as to be almost cliche.
Here is a great guide on the process:
http://www.gunblast.com/FerminGarza-Firelapping.htm
When I purchased my Super Blackhawk Bisley Hunter, it had exactly the same issues you are describing.
- Groups that started off acceptable but quickly opened up with continued shooting
- Leading in the first couple of inches of the barrel
- A slight constriction about mid way down the barrel, under the warning label rollmark.
After reading the above linked article I purchased a firelapping kit from Beartooth bullets.
I would shoot 6, pull the gun apart and scrub the heck out of the bore to ensure I had removed any lead that was deposited and repeat with 6 more rounds.
Every 25 rounds I would slug the bore again and check the constriction.
That process continued for 100 rounds at which time I could no longer feel the constriction when pounding through a pure lead sinker.
After lapping the bore looked like a mirror when you looked down the barrel.
Leading was non existent (with properly sized bullets), regardless of how many rounds were put downrange.
Cleanup required 2-3 patches total. Literally nothing is left in the bore but a hint of unburnt powder flakes and a bit of soot.
Best of all this gun will now shoot sub 3" groups off the bench.
As for a source for bullets, my go to is Beartooth in .432". I am a fan of heavy for caliber bullets and have been very impressed with their 300g WFN GC bullet although the 280g WFN GC and the 250g SWC have been decent performers as well.
You can buy individual pin gagues on amazon:
https://www.amazon.com/Vermont-Gage-...age%2Bset&th=1