Is it best to purchase a single or double roller timing chain? Will a double roller need another timing chain cover? Its a 400hp 350 motor.

A double roller will fit under the cover. On some blocks the chain can hit the center oil galley boss. AFAIK, there is no way to tell in advance. It's an easy fix with a grinder when the block is emtpy. Keeping the grinding dust out of an built engine is a concern. I've always used double rollers but a stock chain might be OK.

What about the single chains like from summit?