Bummer
Yes, I've machined the .083" deep 5.565" rod dish forged piston down to a flat top for use with 5.7" rods. Sounds like you just had bad luck, a block with a thin wall when taken more than .030" oversize. This is one reason I always short fill a block with block filler, it will help avoid that cracking problem.
A good shop could sleeve your cracked hole, then you could add a short fill to stabilize things a bit more and be back in business rather easily.
If you have another good block, I would not bore it to .040" over just to fit the 6 or 7 good pistons you have, best not to bore a 400 any more than you absolutely have too. If it is usable at .020" over I would leave it at that size and just buy new pistons, KB/ICON forged can be had for a decent price. You'll also have a lot more room for more lift with a real 5.7" rod piston rather than the cut down and re-eyebrowed short rod pistons.
I have only done the cut down piston deal with solid flat tappet cams, never with rollers because I know how little piston to valve clearance there is with the cut down piston.
I wouldn't even consider a hyper piston, even if you gave me a free set. Been there, seen how easy they break, cost me a 400 block when I was dumb enough to believe Keith Blacks hype and put a second set in after the first set broke(.030", then .040"). Not worth the cost savings. Nothing but forged for me.
There is a decent set of used TRW Forged L2477F.030 5.7" rod small (.140" tall 6.5cc)solid dome 400 pistons on Ebay right now for less then $200. You could keep the dome or mill it into a flat top. I've used these pistons several times. Listed as a 12.7-1 compression piston by TRW, will make 13.3-1 with a zero decked block and 64cc head.
http://cgi.ebay.com/ebaymotors/Seal...r_Truck_Parts_Accessories&hash=item20b86a977e You'll have a lot more piston to valve clearance with these.
If you do decide the budget means using the pistons you have and just getting two more, and boring the 817 block to .040" over, I would definately consider it mandatory to short fill the block. Wouldn't even consider not filling it at any bore size, really. Short fill to the bottom 1/3 of the side freeze plugs is perfectly streetable, no worry about heat if you have a good radiator and pump.
Epoxy to fix port break-throughs when porting is common, not a problem. I've got a set of Dart Sportsmans that are epoxied on 3 or 4 pushrod pinch areas that were epoxied in 1992 and are still being used by a friend of mine without problem.