I am currently using an old slip tank as a fire drum to heat rocks for sweatlodge. It has hinged door on one side, 8’ vertical chimney pipe 6” diameter. Vent holes cut around the base
Need to reduce smoke and Wood consumption. I am researching methods to convert to a more efficient burning method. I’ve been looking at rocket mass heater, but have to be able to pull out the heated rocks. I’m wondering if an insert would work, and running the exhaust from the bottom out horizontally, then insulating the whole thing with cob or rocks and mortar.
JoBean

Hi Jo;  Welcome to permies!
If I understand correctly, your currently building a fire inside that tank to heat the sweat lodge rocks piled inside ?  You would like not to stick your head in a smoke filled box to retrieve hot rocks ? :)
You are thinking about how to use a rmh to do that ?   It think can be done!
You need a rocket core and barrel.  Your rmh fire built with small dry wood will be super hot with no smoke. The hot air from the 55 gallon barrel is pushed by convection into your current tank,and then vents thru your 6" stack.  You would want to seal up all your vents at the bottom. A several inch thick layer of insulated cob on the bottom and up the sides.  Leave your 6" chimney and expect exhaust leakage at your rock door (as its outside this matters not)
Built properly your rocks should heat up in record time,  I can promise you WILL use way less wood . You will NOT smoke out the neighborhood.
You will want a copy of the builders guide (Ernie & Erica Wisner) to learn how to properly build a rmh.
I think you would want a brick manifold behind your core to plumb the hot air into your "bell"  
Think about it... do some research.. come back here or over at the rocket stove forum and ask questions !
Welcome to the wonderful world of rocket science !