This flexplate shows the additional weight added for an externally balanced stroker 383 crank.
When Chevy originally built the 400 small-block, the longer stroke demanded more counterweight mass, but the engineers decided to add this additional offset weight to the harmonic balancer and the flexplate/flywheel. This is referred to as an externally balanced assembly that was only used on the factory 400 engines. All other small-block Chevys were internally balanced, where there is no offset weight (zero balanced) on either the damper or the flexplate/flywheel. Internally balanced stroker crankshafts move this additional weight onto the crankshaft counterweights. There are two different internally balanced Eagle ESP cast stroker cranks: one that requires at least a 5.700-inch rod and one that can be used with 5.850-inch or longer rods. These cranks add counterweight material, so they require a longer rod for the counterweights to clear the piston skirts when the crank is at bottom dead center (BDC) for that cylinder.
We're going to assume you're using the externally balanced-style crank, which will require a 400-style flywheel and externally balanced front damper. The 400 flywheel is really not much different from a normal flywheel, since the external offset weight is just drilled out of the opposite side of the flywheel. A 383 could use the larger 168-tooth flywheel (11-inch clutch) compared with the Nova's 153-tooth flywheel (10.5-inch clutch). This large flywheel will also require a different starter, one with the offset bolt holes. Check to make sure the block is drilled for the offset starter bolt pattern.
Buying a complete stroker package with the crank, rods, pistons, rings, and bearings is often a good deal because the company has already worked out all the balance details.
As for an externally balanced harmonic balancer, an 8-inch balancer will work as long as it has the offset weight designed into it. There are several companies that offer low-cost, high-quality balancers including Pioneer and Professional Products. Large, heavy balancers are not necessarily a good idea. According to crank manufacturers we've spoken to on this subject, a heavy weight twisting out on the end of the crankshaft tends to add twist to the crankshaft and can often cause cracking problems at the crank snout. A smaller, lighter, 6.5- or 7-inch balancer offers less mass while still producing the desired damping effect.
It also sounds like you may have purchased the crank, rods, and pistons separately in an attempt to save money. Keep in mind that if your rotating package has been pieced together, it still must match the "bob weight" of the crankshaft. The bob weight is the effective weight (in grams) of one piston, its ring set, wristpin, and the small end half of the rod. The crankshaft counterweights are sized to accommodate a given bob weight. The externally balanced Eagle ESP cast cranks require a bob weight of 1,855 grams, which means the bob weight of your components should, ideally, weigh the same. If they weigh less, the balance shop can merely drill holes in the crankshaft counterweights to balance everything out. But if your reciprocating assembly weighs more than 1,855 grams, the heavy Mallory metal must be added to the crank. Adding Mallory metal to your crank for balancing can quickly exceed the cost of the crank itself, so we'd suggest weighing everything ahead of time to ensure the system will balance properly. This is why purchasing an entire rotating assembly from one company is a good idea, since the pistons, rods, wristpins, and rings are carefully matched to minimize balance problems. This was a bit of a drawn-out answer, but it highlights some of the difficulties you face when scratch-building a stroker motor. Hopefully, we've helped you avoid a few of the more difficult problems.