While I have to admit that I have no first hand knowledge of this cartridge, the data in Cartridges of the World make it seem kind of like a BP version of a .357 Max, using a fairly short, heeled bullet.  If I were going to "make one up from scratch" I would go with a faster twist and simply make the bore and chamber dimensions appropriate to the modern cartridge.  I WOULD be very careful to mark it BP only and/or use a stout enough action to take the possibility of a factory level .357 Max being loaded and fired. 

Just my $.02 - Froggie

I own and shoot a Ballard in .38 Long, but not .38 XL.  The bores of the .38 Long and .38 XL are not .358"! They are actually more along the lines of .373" to accomodate the .375" bullets used in these calibers.
The bullet of the Long and XL are a heeled bullet, and thus they sit even with the outside of the case. The XL can be easily reloaded using .357 max. cases, and the .38 Long can use .38 Spl. cases.
I use a round ball cast from a Lee double cavity .375" mold. They make fun plinkers, but I'm looking for the correct mold to cast the heeled 140-150 gr. bullet now. NEI makes one, and I may end up just ordering one from them.
My old Ballard is a 1-36" twist, and this was pretty standard for older guns in either of these .38 calibers.
Whereabouts in Oregon are you Bill? I'm in Portland. You might want to take in the OAC Single Shot Rifle show in January if you're near Portland, Or. in Jan next year! It's a great show!

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Hey Marlinguy!  If I had a special need like you mention for a mould, I would look hard at B&D for a custom job.  Since both the .38 long and extra long would use the same bullet, and since there is a large number of Ballards in .38 rf/cf as well as many just for one of these cf versions out there, it seems like this would be a popular number.  What say you, old seven groove?  Do you have any interest in such a thing?

Froggie

I have a Rook Rifle in .380 Long, which is .38 Long in Americanese.  It has the .38" (as opposed to the .36") groove diameter and I shoot the heeled bullet in it, cast from an Ideal loading tool.  However, the former owner shot .38 Spl. hollow-base wadcutters in it, and he gave me a few rounds with the gun.  I tried them and they did fairly well, as he had reported.  You'd need to buy the swaged Hornady bullets, or cast them out of pure lead, to get the skirts to blow out into the rifling, but they should work OK in your Extra Long if you don't want to mess with heeled bullets.

Froggie: When you say fast twist for a .357, what twist do you have in mind?

How long are the chambers (not cases) on the 38 long & extra long? I just bought a side plate Stevens in 38 (extra?) long. I have not had time to do a chamber cast yet, but a 38 special case won't fully seat in the chamber. 

It might be interesting to put together a new 38 extra long using a new .375-.379 barrel. It could be chambered with essential a throating reamer of 379-380". I had good luck with a Shilen BP barrel (.379 1/20 twist) in 38-55. 
Regards, Ron

I built a 38 extra long on a NEF action using a tj .375 liner and a .357 max reamer. I use .357 max cases and a heeled .375 dia mold. It was actually a good little shooter. I haven't sjot it in a while and don't remember how much powder ended up in the case. It was enough you knew you shot it but still very comfortable. 
A lot of people would have considered this a waste of time and money but I had a lot of fun doing it.
I do agree with Froggie about using a more modern dia barrel. If I did it again that is what I would do, but I wanted to see how the original worked, just one of those curiosity bugs that can't be cured any other way.

Marlinguy,
Would you like to borrow my mold and cast a bunch up then send it back? Would be more than happy to loan it to you. It is a copy of an old original heeled bullet. Don't remeber the exact weight of the bullet. It is pointed and short. Got it from Rapine about 8-9 years ago. Also works for the .38 rimfire reloadable cases with a little patience.
Jon Conard