I don't see point in making a paper internegative with RA-4 paper as you describe -- the paper has contrast too high for internegatives -- why not make film internegative? "Copy film" is not available (if you don't want to use the cine products, process ECN-2), but try the films that are available. They are good for shooting very different scenes with very different lighting, so I would bet they are quite good in capturing the slides, too.
There are three choices then;
1) Internegative of the same size as the original, contact printed from the original, then enlarged to the paper.
+ easy, cheap to make
- a small loss of definition
2) Enlarged internegative to the final size, then contact printed on paper
- expensive
+ the best possible definition
3) Enlarged internegative, but not in the final size. Again enlarged on the paper (e.g. 6x4.5 or 6x9 copy from 35mm)
Something between 1 and 2, but as you enlarge it twice, the definition is more related to the lens, film holders and your ability to focus. Might be very good in the best case.
I would just go with 1 unless you want to make a very large print in which case I would go with 3.
As for copy film, experiment with either Portra 160 or Ektar 100 depending on if you want exaggerated colors or not.
- Bracket the exposures to find the best exposure time.
- If needed, adjust contrast by changing C-41 developer time. For lower contrast, increase exposure by 1/2 stop and develop for 30 seconds less; for higher contrast, decrease exposure by 1/2 stop and develop for 30 seconds more. Or something like that.
- Try to get daylight balanced light from your enlarger when copying the film. I have found something like 30C 15M 0Y to be quite close but this depends on enlarger. It's not that critical, of course you also have your normal controls when making the final print.
