That conversion (in a slightly different case) will work great, it was an original english cartridge called the 375-2.5inch nitro express. Lee Speed conversion rifles were chambered for it.
In todays world, in the U.S., this is still an easy conversion. The cartridge to use is the .38-72. It can be formed directly from Hornady .405 brass; dies are available from 4D dies. This version is about a tenth of an inch shorter than the 2.5 inch one, but uses readily available brass. You can also use 303 british brass, just have to chamber it slightly shorter because that brass isn't quite as long. Or, could get a custom reamer ground, it's called the .375-303. Slightly modifying the sizing die by grinding it shorter might be required if you use .38-72 dies. I've formed brass from .303 in my dies, and they came out dimensionally okay to match my reamer.
I am going to do this exact conversion, and I have the reamer for it already; I've chambered a couple rifles in that caliber. However, instead of using a Lee Enfield, I'll use a P-14 Enfield. That way, it would be worth more than the original parts when it was done, instead of less. P-14 actions are readily availabe in the $100 range, and will work without alteration for this cartridge. just a bit of tuning to make it feed will be required.
dave
