Marlin "micor-groove" and cast bullets.....is almost like "jumping in the deep end" of the pool,haha.Good luck,some will shoot OK at best and others "just never take".
Go with the gas checks,and medium velocities/pressure.Probably start at .309 and be prepared to go up to .310.Big caution right here.......you need to be aware of what the chamber dimensions are,specifically the neck area.By loading a .310 bullet you are effectively creating a "tight neck" situation.Can't say until its measured.Three ways off top of my head to measure are....
>Chamber cast with Cerrosafe,undertsanding the product.....and mic accordingly.
>Small hole gages,again understanding of how these things work and getting good/repeatable measurements.
>Mic spent cases from that gun's chamber.
So,once you have all the safety measuring in line.......whats wrong with a tight neck?Not a whole lot.BR shooter's swear by them.But they come with a price,that price is labor and mindful attention to exacting details.For instance,you can "turn" case necks...which in theory,improves their coencintricity....and run a .310 or maybe even a .311 bullet.Don't take that as an endorsement,just sayin with proper measuring and a nice dose of engineering,theres several ways to approach things.
Be mindful of your O.A.L.,should be mostly cover'd in any good loading manual.Talking here on the relationship of bullets shape(ogive)and the chamber leade.Theres a twist with cast bullets in that,many moulds are designed as "bore riding".You're somewhat limited by the rifle's action on feeding and straying away from published OAL.Spend some quality time reading about jamming bullets.Be extra careful.Just understand the relationship between OAL and chamber pressures.And you'll get some good info looking at pistol rounds at this point.Study how seating depth affects pressure.
Pressure isn't a bad thing....when its understood,read up on it.Which leads to a question,what powder?