Well theres a couple of things your going to have to do.
First is find a set of 351-400 m motor towers so you can drop the
460 in. I believe you can relocate your 360 mounts (but dont quote me on that).
Your going to need a c6 trans for a 460 also, take the internals from the 360 c6 and install them in the 460. This should allow you
to keep your currenty drive shaft lengths.
And dont forget about headers for the exhaust.
Now if you go the c6 route for the 390 you'll need the c6 out of
a early to mid 70,s highboy.

I currently have a 76 highboy with the 360, c6 , Np 205 case with dual Dana 60,s
I changed from a divorced tranfer case to a Connected one. I plan on putting a 390 in next year. But my plan is to do a 76 highboy chassis ground up with a 460. The 460 will be stroked , I m looking to get about 570 cubic inches out of it, this should really haul, it will get a Divorced transfer case with the c6 trans. It will also get lower gear ratios in the diffs to allow some low rpm highway cruising but still haul ***. But it all comes down to your budget and what you want to do.

David