You'd not be the first to under take such a project.
And yes, its perfectly legal as a Bolter is not a real fire arm so you'd not be creating a replica firearm.
This Bolt pistol is made from a G36.
He even did an extensive project time line showing how he did it all:
For a Heavy Bolter I'd start with a 249. Strip down everything you can off it; stock, head shield, carry handle, etc. And simply build off that.
Problem you're going to run into is that the hand held ((Sgt. Harker)) Heavy Bolter had the pistol grip angled back with a second grip so that he heaved it around like a chainsaw. What I'd see you building out of the 249 would be more resembling a pintle or tripod mounted Heavy Bolter.
You might want to go towards a simple Bolter off a G36 similar to the pistol above just build it bigger to the Bolter spec.
But after ALL that said and done; you'd probably want to start it all by getting Verified so you have access to cheaper parts and more information on working with airsoft parts. Changing out barrels is a simple process that just takes some attention to detail and patience ((god damn rubbers that tear, seriously, nothing fucks up your life more than a broken rubber!))