#1). Yes, use .308 gas checks. In theory, if you seat them right, they upset, copper is very soft, to the diameter of the bullet as they go thru the sizing die. If you are not sizing, then the throat gets to do the forming.
#2). Lyman is the big dog in cast bullets. They have lots of books. You should slug your bore and size to groove diameter or .001 over. Hardness of alloy will govern speed. Real heavy bullets will get you all you can out of a SMLE... On line, Hodgdon and others take email questions about use of their powders for this sort of thing. May have some data posted. Assuming your bore is not NRA "moonscape" (ROUGH), this should bring you much satisfaction. Case you didn't know, 303british.com has some data and other lore too. enjoy. luck. happy trails.