Willy, this is very hard to do at long distance, but any rifle that shoots jacketed bullets well should be able to shoot cast bullets reasonably well, say under 2" at 100 yards. If you are paying 80 cents each for jacketed bullets, that's a powerful motivation. I cast my own, but not for .375, and never have more than a nickle a bullet in them.
Are your bullets gas checked? (That's a little copper disk on the base.) If they aren't, you are not going to be able to get anywhere close to 1900 fps with accuracy.
When the pressure is above the limits of your alloy, the bullet flows and deforms, and accuracy vanishes. The loads you mentioned generate more pressure than most alloys will take and it is not wise to reduce charges of IMR 4831 very much.
Shooting cast bullets takes some learning and some tinkering. Almost nobody gets good results right away, but when the cost difference is 75 cents a shot. it's worth putting some effort into it.