Those Speer 300 grainers should shoot just fine in your micro-groove Marlin. Crimp in the forward crimp groove, that's what it is for - it helps set the overall length of the bullet to allow for proper feeding. That cannelure does not need to be either deep or knurled, it's a crimp groove and you should not have any problems with bullet set-back.
Those bullets have been around for a fairly long time. They were designed for the 444 and that micro-groove twist. They should shoot just fine in your rifle. JMTCW. Mikey.