Most of the time you design a wildcat to fill a need not available elsewhere. For the rest of us, just for fun:)
While I don't want to steer you away, I don't think the 7.62x54 is a good base to begin with, only due to the lack of available brass. Graf and Norma have it, who knows how long Graf will keep it up, and Norma is WAY too expensive. But, on to your question.
Sit down and draw out what you want on paper, you'll want to make up a few sample cases just for you so you can compare case capacity with other like rounds. This is where you'll decide to keep or scrap the project. If you find that there is already a factory case that fills the need almost exactly, then there really is no need to spend the 1500.00 or so to build your own. Unless of course you're sure yours will add something to it (e.g. accuracy). 1500.00 sounds like a lot but that's not a far out number for the chamber reamer, die reamer, barrel, dies, and work. I'm giving this number based on the idea of the 7.62x54R. There are no actions or similar rounds designed on it, so the whole project will be custom.
So, you decide to go ahead with the project. You'll need to initially draw (or have drawn) the dimensions for the chamber reamer. Once you have that you can have the reamer made. For the initial gun, I'd use a single shot so you can avoid some of the cost of finding or having made a receiver to mate the bbl to. Offhand, maybe an Encore. One the barrel is made you begin your load development. Which would take me a few pages to fully explain. Feel free to e-mail me if you think you really would like to pursue it, I'll give you as much help as I can and/or point you to better qualified people.