Need help deciding what caliber for new Custom Cowboy

I have a 1991 336CS in .30-30 that I bought from a neighbor for a pretty reasonable amount. It will soon have a date with a custom gunsmith for new wood, octagon barrel and upgrades.

I haven�t been able to find an octagon barrel for the .30-30 on any of the parts websites so I�ve come to the conclusion I will have to order an aftermarket barrel. With that in mind, I can now change the caliber of the rifle especially since I already have 3 other 30-30s.

What I need to know from the forum members is:
What caliber would you select from the list below and if the upgrade is possible and within reason. And if you have a different caliber/cartridge in mind, let me know. I do realize that a different bolt and tube will be necessary. I already have the 1895 Cowboy in .45-70, so that�s why it was left out.

#1 .45-90
#2 .38-56 This is a necked down .45-70
#3 .35 Remington
#4 Or just stick with the .30-30

Your input is very valuable and thank you ahead of time!

Do not thin going to a .45-70? would work with that receiver power load wise. .35Rem would be a good option as that is not so much difference in the load range. I have both and like both but prefer the .30-30 due to the lower cost of ammo. That should be another consideration for your to consider in any conversion.

7x30 waters would be my first choice, but 356 and 375 winnys are also nice.Guess it depends what your going to use it for.

I have always thought a Marlin 336 CB in 35 Rem would be cool to have. 32 Special would be cool and different as well.

With all the love of odd balls, I would look at a 219 Zipper, 25/35 or a 32 winny.

From your list, I'd leave it a .30 WCF. From my list, I'd be flipping a coin between the .25-35 and a 7/30 Waters.

I posted this on the Marlin Owners Forum also and got some excellent information. I plan on upgrading to the .38-55 cartridge since it has the same base as the .30-30. All I'll need to do is change the barrel out, which I was planning on doing in the first place.

Thanks to all for your suggestions!

.38-55 is the way to go