I'm planning to build a 390 also but a budget build, I know your better off with a 360, and I was planning on a 408 but once I added it all up,it would be along time till I could afford it.So I just bought a used low mileage set of Diamond forged 390 pistons on Eagle SIR rods in great shape from a member on this site for $450(should be 10.5 compression). So I'm thinking about an Eagle or Scat cast 4" crank for $300, then all it would cost is the ballancing, machine work for the block, and some rings and bearings, although the rings look useable. My 72 Duster already has a good 318 with Edelbroke intake, headers, and a 650 Holley so I have that covered and I already have a Comp XE274 cam that I can use. I also have a set of "302" heads with 1.92 1.60 valves installed, so I plan on porting them and opening the port openings to 360 size for an Edelbroke RPM Air Gap intake, then down the road get a set of the RHS LAX heads.
I put all this in the comp cams dyno program and got 414HP and 458TQ, with better flowing heads 425hp/461TQ and with a good roller cam 457HP/467TQ . Any way you look at it, even in mild form it should be a nice little motor in my 72 Duster with a 4 speed and 3:91s!!!!
Butch