Do not know of a marlin that is a Model 30-30. DO know of a Model 336 that is in caliber 30-30. IIRC, the barel is threaded, but if it has been on there a while, can be a chore to remove. It DOES require some tools that a hobbyist does not usually have- barrel wrench, and an action block for a vise. Without the blocks, you will likely warp the receiver. if you DO have them, try soaking the joint overnight in Kroil, and heat the reciever (not the barrel) with a hair dryer- trying to get reciever, but not barrel, to expand. Me, I chicken out, and take my rebarrels to my friendly smith- who charges about $35 if I supply parts. You DO have a set of headspace guages for the new barrel, right? If not, you might seriously consider subbing this task out to someone with the tools.
What we have here is... failure- to communicate.