Mine is a 673. It shoots everything very well. Even factory loads. Here's my favorite MOA load and it packs a whallop! Stole it from Ken Waters:
.350 Remington Magnum - Lot "A"
Primer: WLR
Powder: IMR 4895 55.0 Gr. (See Below) Listed as "Near Max."
Bullet: Speer Hot Cor 250 Gr. Spitzer
OAL: 2.900"
Case Usage: New Remington - Peters
Remarks: Ken Waters "Pet Loads" p. 411 says "near max., good load." He's right!
IMR 4895 is definitely a good powder for this round.
Start lower and work up!