IMO, the best scope rail is the standard H&R, it has the most cross slots and mounting options without modification, I've never had a problem with any of them on many, many rifles, I've had to chamfer the top of the front end for mounting scopes real low, but I don't see that as a "problem". The 1" longer Ultra rail would be my next choice for mounting big scopes on a varmint rifle.
Recommended spare parts are the transfer bar (lifter & striker), firing pin and spring, hammer spring.
Chronic short throats in the 45-70 barrels has been a major gripe on many barrels for years, but not all are short throated, you'll know when you go to check COL for most of the 350gr and bigger bullets in your chamber.
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