With a 4.125" Bore / 3.76" Stroke, 402ci Closed Chamber 101cc head and OEM Stk .019" Shim Head Gasket, you require a Closed Chamber Piston with a dome between 24 to 28cc's to give you between 10.5 to 11:1 Static Compression Ratio where you can run a Strong Cam and still use Pump Gas. I would lean toward the 10.5:1 side of the equation and choose a cam with a duration to produce between an 8.1 to an 8.2 DCR.
Either use the .019" Shim Head Gasket giving a .044" Quench or 0-Deck the Block and run a .039" Composite Gasket.
Look at the Speed Pro Forged Line of Pistons at Summit. Closet I could find for that 101cc Closed Chamber was this Piston ZL2240NF - 21 cc Dome to give you a 10.1:1 which is a Safe Static compression for Pump Gas. You have to consider that you have to match available Head and Piston Castings to achieve your desired Static Compression. If you are running verily Tall gears like 3.31's or 3.08's, I would recommend the Comp Cams XM270H-12 1600-5800 270-286 Adv [email protected]
.544/.547 112° Cam to give you an 8.2 DCR to Maximize Torque in a Small Displacement BBC perhaps the XM278H-12(wicked 10:1 454 Cam) with 3.55 and lower Gears.
Play with P. Kelley's DCR Calculator here: http://www.empirenet.com/pkelley2/DynamicCR.html
and Comp Cams Camquest here: http://www.compcams.com/Camquest/
Checkout Mortec Site for BBC Casting#: http://www.mortec.com/bbc.htm
The 396 and the 396 + .030" 402 are the same engine where all 396's had closed Chamber Heads but 402's had lower Compression and used larger Volume Open Chamber heads. So what you are doing is putting a 396 Head on a 402, which likely had open chamber pistons, and must use Closed Chambered 396 Piston.
You can NOT use the 402 Open Chamber Domed Piston with a 396 Closed Chamber Head but can use Close Chamber Domes in Open Chamber Heads; however, you have to mock the Head Assembly to the Block and Scribe the Bore to the Cylinder Head and Open the Chamber as the Pistons can strike the outer Diameter of the Combustion Chamber, plus I would confirm Clearance using Plumbers Putty.
You will find that most Hi-Perf aftermarket MK IV Piston Dome Volumes are quit large and geared toward the 113, 118 and 122cc Open Chamber Castings; however, you do have a better flame with lower domes. I would say that 10.1:1, ZL2240NF/290, Combination above would be about Ideal with that XM270H-12 Cam on Pump Gas; besides, nothing sounds better then a 112LSA Cam ;o)
Always CC the Heads to Confirm Chamber Volume as they are not always Specked as GM Advertises.
Top her off with a 750 Holley/Edelbrock RPM and Headers - 7/16" Push Rods and I recommend 2" ARP Rocker Studs / Roller Rockers like Pro Mags or Harland Sharp.
Your looking at approx 430 to 445 Hp with a HFT Cam perhaps 460 with a SFT Cam.
Everything depends on what you intend to do with your ride - Tranny/ Gear ratio's... etc and how much you want to spend. I given you a Low Cost Solid Performer solution that should last for Years on Pump gas without much Maintenance.