Wow, new to fabricating? You have bitten off a HUGE project. The body mounts are the easiest part. You would place the body into position and build the mount up to the body, that simple. Trying to make the mount using measurements off the old frame "could" be done but not knowing exactly where the "new" frame may hit under the body and that sort of thing there is no way I would try that.
Honestly, this is a HUGE job which will require a LOT of fabrication and cutting up the inner fenders and rad support. This is before you do things like mounting bumpers which are another project.
It's like the old engine swap lingo, people will ask about the mounts, making the mounts as if THAT is all that is needed for an engine swap, the mounts are the easiest part. Often the complication comes in the firewall clearance, rad location, rad outlet location, steering box clearance, steering column, exhaust/headers, there are a PILE of stuff that may need attention for a "simple" engine swap. Same is true changing a frame.
Tell me, what is wrong with the old frame? There are PILES of kits available to upgrade that frame that require no fabrication, or little fab so you can get your feet wet. And remember, that frame you have under that car, it's the same as a 53-62 Corvette, it has the exact same suspension (rear springs are different is all) as the Corvette, so it isn't "that" bad.
By the way I am looking to buy one of those front crossemembers to use in my truck if you want to sell it. Isn't that funny, I am going to upgrade my 48 Chevy pickup with the suspension from a car like yours.