My JES 3 groove rebored Marlin likes both .377 and .378 sized bullets with a slight preference for .378. Started out with the Lee mold and switched to an Accurate 38-250B plain based mold, much easier to cast good bullets with. The rifle likes both bullets though with the lower velocity loads I typically use. 10 gr Unique or Universal are great for plinker type loads, 31 or 32 gr 3031 makes a fairly stout hunting load. Both are really accurate in my rifle.
I use the RCBS Cowboy dies with the larger neck expander to keep from resizing the softer bullets I use during bullet seating.
I've only used the Starline 2.082 short brass and it works great in this setup.