THE KNOCK !! shes out now what?

Wyoduster, I would start by looking at the rods, both ends, to see if there is any play whatsoever. Even a little play in anything bolted down would indicate to me where the problem lies. I would suspect a rod journal bearing, either spun, worn gone or scored and burned up. I'm sure others will also have some suggestions, but I would start with the most common cause of a "knock". Hope you find it and it did no other damage, I would like to see the 340 go back in. Too bad you're not closer to Michigan, I know of a nice running 360 here that a buddy just bought for $300.00. Hope you find the source of the knock and get it all sorted out. Geof