First off, never thought Id do any retro stuff, BUT...
A buddy had a fixed carry handle DPMS carbine that he wanted swapped to flat top. I swapped his upper for him and got him a taller front sight post.
So now I have a take off fixed handle upper and a shorter sight post. It hit me... RETRO.
It doesn't have to be balls on accurate, but triangular hand guards would be neat. Where do I begin? Where do I get retro parts?
For the stock set I would look at uscorereplicas.net. For the barrel and other small A1 parts like the slip ring and flash hider I suggest ar15sport.com the barrel I bought is scary accurate and the dude I talked to was helpful and seemed pretty stand up. An LPK from any decent vendor will due. Same goes for the buffer and buffer tube assembly. For full disclosure I have no experiences with US Core, but if you look around all their feedback on here is pretty stellar. There is also a sticky thread that has all sorts of links to websites that carry retro parts.
And sorry if those links don't work, I'm still learning this site.
I have no experience with US Core but they are EXPENSIVE
Even though you already have an upper, I suggest ordering a parts kit from RTI, they are $350 and include everything but the stock, trigger guard, barrel, lower and a few springs.
If you decide not to use some of the parts you can sell them for a premium.
You can place a WTB ad in the EE for anything else that you need.
This route will get you the most correct build for the least amount of money IMO.