I'd stick with a stock 4L80E converter and get the flat flywheel and spacer for it. My stock 4L80E converter is a bit tight but you can get slightly higher stall stock converters. If it's for towing, I'd stick with a large (stock diameter) for the added clutch area < that's purely my understanding and not anything I've read haha

http://m.ebay.com/itm/Flexplate-for-...%257Ciid%253A1