If you have more 440 blocks and all of your parts are for RB engines then I'd build another 440. You might just think hard about ways to reduce detonation and reduce problems for the bottom end. Aluminum rods might help, double check the balance job on the rotating assembly, careful look at the tops of the pistons and the combustion chambers, double check your AF numbers, etc. You might have a distribution problem and maybe one cylinder is running lean and you don't know it. 6bbl systems are fairly famous for having problems like that.
+1
The major problem with stock block main problems are with balance, anything that disrupts that (bob weight, fuel mixture, timing, etc.) will cause the bottom end to do ugly things.
Here is an excellent article on the subject
Engine Balance 
