Cool, another Okie, where are you located?
I got a Mopar buddy as well.
First off, your buddy needs to sign on here for all of his Mustang info, but for his 390, he needs to go to FordFE.com.
Depending on how much he wants to spend, I'd stroke the shortblock with a Scat crank kit. Comes with a 3.98 (which is 428 stroke, a 390 is a 3.78 stroke, or a 4.125" stroke, or a 4.25" stroke cast crank that will withstand up to 600HP easily. The kit will have rods, either I-beam or H-beam, the 4.125" and 4.25" strokers have 6.7" rods with BBC journals as that's what those two cranks are cut for. Diamond makes a great forged piston for these setups in all the common overbores.
Since he wants the engine to look stock externally, I'd suggest sending the heads off to Les Schmader (I'm assuming they're of the C8AE-H 14 bolt hole variety) for port work. He's REALLY reasonable and does excellent work. He also might find someone on the FE forum that has upgraded to E-bocks or BT heads and has a set of ported iron heads for sale. Scott Vincent is another good head porter and usually frequents the FE forum and the 385 forum (BBF, 429/460).
Jay Brown is a member of the forum and won the Hot Rod Drag Week 2005 BB N/A catagory last year with his green '69 Mach I and is gunning for the BB Power Adder catagory this year in the same car with a Procharger. Jay also has a engine dyno in his chicken coop (I know....) and recently did a pretty extensive intake testing session using most all of the dual plane single 4bbls and several single plane ones as well. He had a warmed over 428 with ported heads that he tested them on, which should be pretty close to what your buddy will end up with, so the testing should be valid to him.
The biggest thing about FE blocks is to have them sonic tested as sometimes the cylinder walls can get pretty thin on an overbore. The best free way to test them is to do a drill bit test. You pull the freeze plugs and stick the shank of a drill bit between the cylinders, try and get from the very bottom up to the top. Figure out what the biggest bit you can fit in there is and since we know that the cylinder spacing is 4.630", you check the bore, say it's .030" over, so then the bore will be 4.080" (stock bore on a 390 is 4.05") so we subtract that and get .550" which is both cylinder wall thicknesses plus the gap between, so if you could fit a 17/64" drill bit in there, subtract that (.265625") from .550' and if you divide the remaining number by two, theoretically, you have about 9/64" walls left (.1421875"). Now this is assuming there is no core shift at all and everything was 'perfect'. If you're putting too big of a drill bit in there, or the block has already been bored out quite a bit, he may need to look for a new block. Usually .030" to .040" is safe, over that and you're pushing it as a rule. That being said, I have a .060" over 390 (4.11" bore) that, barring core shift, I can bore to a 428 bore size (4.13") and still have .125" cylinder wall thickness.
Generally, about a .110" cylinder wall will hold up in a street driven car without a whole lot of HP or compression, but I think .125" is about the minimum I'd use on a drag engine or something that makes a bit of HP. A good stock block to get is a 391 big truck block. These have a steel crank in them like the 427's had. This can be sent off to Accucrank in Arkansas to be offset ground for more stroke, but since a Scat cast crank is less than $500 and the 391 crank is used, most just go Scat. The 391 block has a larger distributor hole, but this can be bushed. A 391 block can typically take a 4.13" bore or better pretty easily as they were cast with really thick cylinder walls. There are also some 428 industrial engines floating around that farmers used for irrigation and typically ran on propane. These are usually good engines as well.
