Welcome aboard W.E.!!
As far as load data and which to use for the .45-70, the NEF/H&R SB2 frames built 1999 and later(see build date info in FAQ) are safe to low modern rifle or Ruger #1 data, which depending on the source, is somewhere in the 40-50kcup range. But recoil is usually the limiting factor depending on your sensitivity to it, heavy or fast loads in .45-70 develop a lot of recoil, add the steel butt plate and lightness of the BC and you add to the recoil factor of the caliber itself. H&R's recommendation is to use levergun data, but several of us have used low Ruger loads with no ill effects accept brutal recoil, so let your body be the best judge!! :eek:
Lockup and latch engagement is important, it can make or break accuracy, again the FAQ and Help sticky under Accurizing and Barrel Fitting can give you the answers you seek. I've increased spring tension in several of my frames by installing a 2-spring combo that Fred M developed, he discusses it in his Accurzing link in the FAQ.
Walmart sells the BC about as inexpensive as anyone and their supplier, Sports South, ships them immediately, so they never show them in stock, but they're one of the better choices since Sports South is a big wholesaler and gets them in about as much as anyone else. They show them out of stock right now, but that's usually the case.
http://www.walmart.com/catalog/product.do?product_id=3138388http://www.internetguncatalog.com/results.aspDunno how far you are from OkC, but Sportsmans Warehouse sells them for $300 and regularly stocks them if they're close enough.
http://sportsmanswarehouse.com/stores_all.htm#okHope this helps to get ya started.
Tim