Any "stock replacement" head gasket has the appropriate "reliefs" for the chamfers. We prefer the Victor/Reinz, but of course, Felpro is very popular. Unless static compression is over 12:1, there is no need for "special" head gaskets. Stock ones hold up fine. Just be certain you use the correct octane rating for the compression you have. You will have better service and good parts available at your local parts outlet. No need for "mail order". Pontiac parts are NOT rare. If they don't have them "on the shelf", they can get them in one day, as ALL the "big" warehouses stock them.
The chamfer in the block is to "unshroud" the intake valve. 396 and 351C also use the technique. The small loss of compression is far offset by the improved flow. In olden times, we would actually sleeve all the cylinders to rid them of the chamfer (for a "class" race car). Today, we make it a little wider... Due to the necessity of lower compression ratios for street engines in recent years, we've learned "flow trumps compression".
The 1/4" between the cylinders is no big deal. The aftermarket blocks, with their 4.350-4.4" bores, have nothing more to "leave", yet seal fine under "race" conditions. While it's true, the Pontiac has "only" ten head bolts, consider that those ten are 1/2", not 7/16", and they're located in the best possible locations. Note the center bolt on the lower row is "closer" to the middle of the block than the "outer" bolts. This is where the clamping force is necessary, and also what makes the Pontiac different from other engines using ten head bolts.