If you measure the rim cut out, you'll probably see its up to about .074" deep, the rim of a 45-70 case is ~.065" thick, depending on the brand.
Hopefully the extractor will hold the headspace to a minimum as an ejector does.
I thought about JB Welding a stop under the ejector post to trap the rim at zero head space, but since I've never seen a bad primer issue, I've not taken that step, but I think it would work with an extractor barrel also.
Tim