"Dose anyone know what the bore spacing is on the gmc 270/302 inline motors?"
"I have 4.463 bore spacing, 228-302cid"
Not correct, the distance between adjacent cylinders is staggered, just like the 216/235/261 and Buick L8 (any many other pre-WW2 inline engines). There's a wider distance between a cylinder pair across a main bearing bulkhead (#2&3, # 4&5: about 4-9/16") than between cylinders sharing an intake port (#1&2, #3&4, and #5&6: about 4-3/8").
If these are accurate, the average bore spacing for all 6 cylinders is about 4.45", but that's not the actual distance for any pair. I suspect the dimension given above was arrived at by measuring from one side of the #1 bore to the same side of the #6 bore, viz. the total, which was then averaged.