The 400 uses a 4 1/8" bore, and a 3 3/4" stroke. The center cylinder walls are siamesed together due to the large bore size, and there must be a couple of extra holes in the cylinder heads and the block deck to allow coolant to circulate past the cylinders. An alternative that gets used often is putting a stroker crankshaft into the 350, a special-order item that puts the 400 stroke into the more reliable 350 block. I think the resulting displacement is about 375 cubic inches. The 400 crank is not a bolt-in swap into the 350 due to different main bearing diameter.
The flywheel and harmonic balancer for the 400 is different, but the stroker crank for the 350 is internally-balanced. Check out Scat or Eagle crankshafts for the long-stroke 350 shafts. Of course, you could buy a NASCAR crank from King's in North Carolina, with any stroke you specify, but his price is about $2,800.00 for a basic setup, with an extra thousand for rods, and another grand for pistons! Nothing like a $5,000.00 rotating assembly!
Jerry
