Several places advertise them but no one I've found has them or a firm date when they will.
My 408 has stock caps and ARP studs and no fill, upon teardown it showed signs of cap walk on a couple caps and one cap is loose in the registers. The main and cam bearings showed odd bearing wear from the front to back. The main bearings were worn on the bottom in the front and the wear moved more to the top at the #4 journal. This indicates to me that the block was flexing and I consider myself lucky it didn't bust a cylinder and ruin everything.
A girdle MAY have helped with the cap walk buy IMO would do nothing to strengthen the block.
My current build uses a R1 block, when compared to a production block it's easy to see why the R series blocks are stronger.