Art - One source I use a lot for hardware is McMaster-Carr (
www.mcmaster.com). I checked and they don't have any 8" long 7/16" carriage bolts but do have 8" 7/16"-14 partially threaded hex head bolts ($13 for 5 bolts plus shipping).
Using Kube's recommendation of welding you could the top unthreaded part of a carriage bolt and add the bottom partially threaded section from these bolts to create exactly what you need with just one weld joint. I'm sure the original bolts were only threaded on the end.
I've done this with good results on the bed frame u-bolts for my truck. Just took a bit of welding and grinding but they all came out just fine.
Sorry I can't confirm the bolt size for you.