I am building a 400 sbc.  Some guys say that a 5.7 rod is better but cant explain why and some say a 6.0 inch rod but cant say why.  So I am coming to you guys for the final answer.  I don't want to have to do any clearancing in the block. Which rod and why, this is going in a awb chevy 1/2 ton with 700R trans, afr 180 heads, forged pistons, air gao intake and a holley carb. I have a smal base circle roller cam.  

Depends really on what you can/want to afford. If it is a mild engine where you have no concerns with I-beam rods being durable enough I would stick with 5.7 rods since they would be cheaper, if you can afford longer rods there might be a slight power increase, but it's minimal (1-3% or something like that). The length isn't as important to clearance issues as the style of the rod, capscrew rods help gain clearance. The stock 5.565" rods are clearanced for cam interference, but 350 rods and larger-than stock cams dwindle away this clearance, so you surely have to check, and might have to clearance the block anyway.

As long as youre replacing rods either 5.7 or 6 inch is fine. The thing you need to remember is the piston needs to be matched to the rod length other wise these long rods on a 400 will push the piston out the top and into the counterweights at the bottom of their strokes.  The small base circle cam should provide adaquate clearance, but just in case some grinding on the rod becomes necessary I' go with cap screw attachment rather than bolt and nut of the rod cap. Either I beam or H beam configurations are fine. However, your power level doesn't really require the additional expense of H beams.

I'm not convinced that forged pistons are needed at the modestly high power levels this engine will operate at. They are a pain to warm up on a street engine. You really have to be careful to bring them up to temperature before moving out, otherwise you get a lot of skirt slap which is hard on the piston and really beats the daylights out of these thin wall cylinders. If your'e using a blower or nitrous then the answer is forged pistons and the patience to warm the engine up before dumping loads on the motor. or the money to rebuild frequently. This is a much bigger problem if you live where the weather gets cold.

Bogie

what is the power gain over forged or cast, and if not forged then exactly what piston, name and part number, please  

what is the power gain over forged or cast, and if not forged then exactly what piston, name and part number, please

I don't think forged, cast, or hypereutectic make different power, it depends on what you want out of your motor... what you need. You probably will never "need" forged pistons if you don't plan to run a power adder... like NOS or a blower or a turbo. Forged pistons are (or can be) considerably lighter for racing applications, and a lighter rotating assembly will accelerate faster... but I can't even guess on the size of any gains.

I will be starting my 406 soon,  but I'm building it to be a low octane, low rpm, economy intended daily driver... yet with plenty of torque to tow occasionally.  The heads I will be using are the Vortecs with 64cc chambers, so I decided on using the Silvolite pistons (summit pn KB-126030-8). They have a 30cc D-dish, for a 5.7 rod*, and are $257 a set. They will give me about 9:1 compression.  ( * I scored a used set of H-beam Eagle rods for $100 )

If I recall correctly, the AFR heads have 68cc chambers, so you may want to seek a smaller dish, but I strongly recommend seeking out a D-dish piston.  You can play around with compression ratios with several available online engine calculators, like this one from BRC...

http://www.brcpistons.com/enginecalc.htm

Just do your homework... do you want an engine that can run on 87 octane, or do you want a more powerful higher compression motor that requires 92 or 93 octane? With the AFR's efficient chambers and a D-dish piston, you can probably safely run 87 octane with 9:1 compression, run 89 octane with close to 10:1, and run 93 octane with nearly 11:1...  but those are very vague "guestimates".

I have the 74 cc competition heads, and I have prived a scat rotating assembly fot right at about 1100 bucks, Cast steel stroker lightweight crank, I beam 7/16 cap screw rods, forged pistions, rings, main and rod bearings with internal balance, if I get standard steel crank it will be 1500.  

Well then,  this should make it an easier choice for you as far as 5.7 or 6.0" rods...

The 3.75" stroke of a 400 with 6" rods puts the wrist pin up into the oil ring, necessitating a strengthening support under the oil ring ... not so good for the street and longevity.

As Gibs ( and Bogie ) have said, the 6" rod may give you 1-3% more power over a 5.7" rod,  but I think it's safe to say for a street vehicle, the 5.7" rod is the best choice.

If it's not a street car, then the 700r4 isn't the best choice...

it's all about application...  what you'll be using it for.

The guys at scat convinced me to go with the 6'' rod, so I will try that.  My cam is a comp cams 12-422-8 with a 1.6 rocker.  I want the lift but to keep duration at about 274 and operating range from 16-1800 to 5400, cause I am using a 2500 stall.  

If you're limiting the engine to 5,400rpm then there's no need for a 6" rod.  However, do replace the too short 5.565 rods for 5.7s.