Hi Dave
I have the spartan 45/70, as well, and just love it with reasonable loads.. its WAY easy to adjust regulation. I scoped mine, and its a true 4wheeler doublegun

The De Haan is a better grade of the same type rifle.. better fit and finish is practical terms, in REAL terms, they ordered to a higher spec rate than generally done.

i would NOT do a 38/55, as 375 winchester cases fall in, and are much higher pressure. 9,3x74 or .375x74, and Dave Davidson, www.ch4d.com has dies for the latter.

I sleeved my barrels, though WHEN (sigh, i am a glutton for punishment) I do my next one, I am going to tear down the monoblock and start from "scratch" ..

my advice, for what its worth, is if you are going to use the barrels as monoblock, then if you want the SIMPLE approach, go .003 larger than where you plan to cut off the barrels to monoblock. long answer is to make the foreend measurements about the same, and then taper the fronts a little more.

its all about your tastes, fwiw