Like your fan issue, water movement & heat transfer inside the boiler will likely be a fuzzy picture until you actually get burning with it. I think the locations you are planning for supply & return are the best for those purposes - but you also might end up needing to install a separate mixing pump to prevent localized boiling at possible hot spots above the firebox. I can picture there not being a lot of water movement in the top front part of your water jacket. Your system flows etc. will also play a part - so if you can plan for that eventuality now, adding a second pump later will be easy. Maybe extra ports top front & bottom rear? I know I have heard of guys with Garns adding secondary mixing pumps - they generate a LOT of heat in a big hurry though.