Welcome
@Seetribe , you have made a wise decision to post your question here...it will save you much heartache!!
First of all, don't walk, RUN! from that SF3100! Do some research here on the newer FireChief/Shelter furnaces (same furnace) they're TERRIBLE!
Second of all, your ducts
need to have rise...going down with a supply duct is a no-no on a solid fuel furnace (wood/coal)...and a
bad idea...BUT, if you have to do it, there is a way...look for an emergency heat dump. Here is one made by Yukon...there may be others out there still too.
https://wood-furnaces.net/safetyheatdump.aspx
It has a door that is tilted down, but held closed by a spring and a fusible link...if the power goes out, or some other blower related failure occurs where the furnace/ductwork is going to overheat, the fusible link melts at a very specific temp, allowing the air to flow again and the firebox to shed its heat. I have one on my system and it works well...I've never needed it, but it gives me peace of mind when I pack the firebox full and go to work with my family still sleeping...I could just see a commuter crashing into a pole and knocking out the power right when my furnace is making the most heat...oh and all my ducts have rise, I just wanted the insurance.
- You will need to install a plenum on the furnace to use a heat dump, but that is not hard...I did one on a Tundra before they came with them...I just bent a lip on the bottom of the plenum and used pop rivets into the furnace air jacket. Speaking of Tundra...the SBI Tundra II would be a much better choice of furnace...or HeatPro if you need something bigger...Or, if you have the budget to pull it off, you need to check out the Kuuma Vaporfire 100...Cadillac of wood heat, and the only legitimately "2020" certified wood furnace in the USA. (broken link removed) you would not be disappointed!