APUG isn't going to like this answer, but I would suggest sending them off to scancafe. For a small (<1000) batch of negs, it will be affordable (certainly compared to buying a good enough scanner) and you will get professional results. Make sure you take the option to get the raw 16-bit scans so you can make adjsutments yourself when necessary, and you can consider the copies archival. They're probably not even C-41 negs and they are likely to have faded past the point where they can be corrected in an optical printing process.
By all means if you're setup for RA-4 and competent with it, give them a go. However, learning colour printing on some precious yet delicate, old and faded negatives from the wrong process sounds like hell to me.
Most optical color printers (up until the latest laser versions) are still in operation and came with a set of legacy masks. the print size can be adjusted by a knob on the right feed roller to account for length of print, and the lens can be moved to give proper magnification. Even some laser printers can do this.
OTOH, automatic optical enlargers can handle this with no problem. I have masks for my enlarger for sizes from 35mm to 4x5 in several formats and can get more just by ordering them.
Kodak negatives from the 60s and early 70 are C22, not C41, other brands used a differnt had other odd film types. I have a lot of similair color negatives from the early 60s to the mid 70 at which point my parents bought a Polaroid (sp?). They were kept by my parents in Los Vegas for a number of years in a uncooled storage room. Some were Kodak, other GAF, others not marked,may have been 3M which was the house brand at K mart where they shopped in the day. I attempted to print myself using R4, but the dyes had shifted color. I took some to a local color lab who color corrected with a minlab, others I scanned and printed at a Target or printed at home, cheaper at Target. My negatives are all 35mm just a 126. If your negnatives were not properly stored you will be better off buying a scanner and correcting in post rather than printing R4.