Matabele,
Much depends on how you load it but a 30/30 is pushing the limit for a conversion caliber. I know that many makers make rifles in bigger and more intense calibers, but those are not conversions from a shotgun. Your donor gun sounds just fine for something like the 30/40 Krag or even a senseably loaded 30/30.

The problems start when some unknowing person sees that it is 30/30 and figures it is all right to load it up with some of the new, more intensely loaded cartridges that shoot flater. Some of these new "extended range" loads have to be really pushing the limit.
All three calibers you mentioned will work just fine. With the 30/40 being the mildest, I would lean in that direction, except for the fact as already stated, that brass will be hard to come by.
303 used to be dead cheap and available anywhere, but I don't suppose it is anymore. Given all that, I would think a 30/30 loaded senseably and clearly marked with the load info would be a good way to go. Any of the three would make you a good treking rifle for plains game. Bob