There are a couple of ways you can go here. I presume you are replacing the 228 with a 302, so you want to keep the 228 bellhousing.
First is to find a late 248 (54 or 55/1st) flywheel with the 6-bolt crank bolt pattern. This flywheel uses the smaller clutch, which will work fine on a 302. You don't need the big 11" truck clutch used on the heavy trucks that came with 302s. You might have to swap ring gears to work with the 228 starter. A 55-62 270 flywheel will also work, if from a pickup. It will have the same small clutch pattern.
Since these flywheels can be hard to find, another way to go is to modify a V8 flywheel to work on the GMC crank. Be sure to find one where the casting number ends with "N", that denotes a nodular iron flywheel. This requires machine work to open up the crank flange diameter and re-drilling the bolt pattern to match the GMC crank. It's also a good idea to add a couple of dowel pins to the crank and corresponding holes in the flywheel. This will allow you to use any of the huge choice of aftermarket V8 clutches - both diaphragm and 3-finger style - that are available for the small block v8.
A couple of further notes - if you plan to turn this 302 more than about 3500 RPM, it will want to shake the flywheel bolts loose (and damper pulley too, but that's another subject). Use ONLY the stock 1/2" GMC flywheel bolts and either loctite or epoxy them in place. Safety wiring the flywheel bolts is not a solution here, the heads are too thin to be drilled. Also, the older style truck clutches have a lot of clamping force but won't tolerate much RPM before the retaining tabs distort, or the pressure plate flies apart from centrifugal force.
