I have been messing with GMC engines for almost 70 years. Parts are getting really scarce. I suppose there isn't that much interest in GMC any more. Especially the 228 and the 246. I have two trucks that were born with a 236, but were retrofitted with a 248 and a 302. I have never seen a 256 in real life. I think it will be very hard to find something to use in a 228 and 248 1952 and older with the big crown pistons because the combustion chamber is so large. If I were to rebuild a 228 or 248 now, I would be afraid to bore it for fear of not finding pistons. I would just leave them standard. If the pistons have worn skirts, I would consider knurling them. That worked for us 60 years ago and it would still work. I am going on 82 years old. I don't know if I will ever build an engine again. And I have lots of NOS parts in case I ever want to build one.