The old school way of building a 383 out of a 350 was to use a 400 crank with the mains turned down to the 350 journal size with a 5.56 inch 400 rod and a 350 piston. While it works it's a lot less than ideal on a couple fronts.
1) The combination length of half the 400 stroke, the 400 rod and the 350 piston has the piston .055 inch lower than in the 350, this gets you into either sacrificing a lot of squish quench due to the excess distance from crown to deck or getting into a lot of deck milling to a point where I feel the deck’s strength is jeopardized.
2) The short rod increases the jerk rate on the piston as it moves from TDC intake increasing the probability of a rod, pin or piston failure. The short rod increases the piston skirt thrust loads which upsets the ring seal. The 400 was never a super high mile engine unless driven slowly. While most people see the sharp rod angularity as a power robber due to the forces the skirt exerts on the bore, the real gremlin here is the loss of ring seal that lets blow-by into the crankcase in one direction and shoves oil into the combustion chamber in the other. This makes the engine an oil burner long before the wall clearance would indicate.
Getting to a 383 with a modern kit where the crank is made to 350 journal sizes to start with, the counter-weights adjusted for the proper masses, the choice of 383 specific pistons that will support the use of 5.7 or 6 inch rods is a much superior layout than when we cobbled these things together back in the 1970's and 80's.
Additionally and very importantly is that with a modern cap screw rod, the whole damn issue of clearancing for the cam and often the bottom of the cylinder spigot and pan rail is totally removed. Therefore, the OEM 350 style bolt and nut rod is not strength compromised by grinding the bolt head and rod shank for clearance to the cam. At the other end your chances of discovering coolant by grinding through the cylinder spigot nor leaking oil through the pan rail gasket are eliminated. So going modern is a very huge bonus compared to old school ways of hacking things together.
In terms of pieces that can be bought individually or as a kit I'd recommend the following:
- Crankshaft SCAT CS383P6, 2pc seal, 6 inch rod, internally balanced, cast steel, drilled throws which gets the counterbalance weight down. This latter feature makes the crank simpler and cheaper to balance and results not only in a improved bearing wear and lower stress on the blocks 3 center mains (thus the purpose of 4 bolt mains on these guys) but the lighter crank accelerates faster as power doesn't have to be used to overcome its Polar Moment of Inertia which makes that power available to move the vehicle faster.
- Rods either the SCAT SCR6A or the SCR6A7. Both of these are 6 inch rods that use bushed floating wrist pins rather that the butt cheap factory press pin. The floating pin in the rod not only does away with the forces of pressing the old pin out and hoping the guy at the machine shop supported the rod's pin end adequately so as not to bend or twist the rod; plus you don't reheat the end of your alloy steel, heat treated rod to stuff the new pin in. These are huge positives that increase the margin of safety to the engine's bottom end when you put your foot in the carburetor. Both of these rods are cap screw rods and are clearanced for the cam, bottom of the cylinder bore, and the pan rail. Both rods locate the cap with dowels and provide extra meat outside the bolt boss which does an excellent job of reacting the stretch loads when the piston/rod assembly is jerked down on the intake stroke. This helps to keep the rod cap from pinching the bearing which in turn wipes out the oil film, this is the first step to starving the bearing for oil welding it to the crank thus spinning it and eventually busting the rod just above the big end. This is so much goodness as you can get for the price of these rods it's amazing. The SCR6A7 is a very prettied-up version of the SCR6A for a little more money, it has design features that have a lot of similarity to the Crower Stroker Sportsman and the Lunati ProMod which make it a mighty good deal for your money. Either of these make an excellent choice for what you're thinking of building.
- Pistons to a big extent this depends on your head selection and whether the block is decked or not as these things affect combustion efficiency and detonation tolerance. Never forget that the piston is the bottom of the combustion chamber so its shape is most important. In your case I'd recommend a Keith Black hypereutectic casting; the KB164 for a 6 inch floating pin rod, this produces a 9.6 SCR with a 64 cc chamber, it is a D dish design which puts the function of a flat top under the heads squish/quench step while managing the SCR with a dish under the valve pocket. The KB122 is a flat top that produces 10.9 with a 64 cc head. Keep in mind that compression varies with deck clearance, gasket thickness and diameter, piston dishes or domes, piston compression height, and combustion chamber volume.
I don't know about finding these choices in a kit, let alone a balanced kit. Most kits just don't pick all the stuff I like, so I don't look too closely at them anymore. It seems that these kits often include one part or another that is some floor sweeping they're trying to get rid of that nobody really wants. Since I don't like blown up engines, I'm pretty picky when it comes to parts and machining.
You can find this stuff at Summit, Jegs, Competition Products, Skip White and many other places.