I've only had experience with an early '70s Topper 30-30, it wouldn't come close to fitting any of my 30 some frames that I had at the time, but others have had them lock right up on their modern frames, so I think the tolerances in frames and barrel of H&R inc firearms was pretty wide in comparison to modern H&R1871 firearms. As with any H&R firearm, the only way you'll know is to try it, if it's close and you don't feel comfortable fitting it per the FAQs info, any competent smith should be able to do it.
A note of caution, read the warning in the FAQs about using high pressure rounds such as the 223 Rem on old cast iron frames, while it may work for a while, it's certainly not a good idea and not worth the possible catastrophic failure that cast iron is subject to.

Tim