Now that is not a good plan. Sorry... The exhaust plume is very hot and will radiate/conduct/convect a lot of heat to the airframe in that configuration. You will melt/burn/destroy the lower airframe which is also your fin unit. That is a disaster waiting to happen. Additionally, in that configuration the lower airframe is also the rest of your nozzle. It makes for a severely overexpanded configuration, and you will be losing thrust. You will get hot incandescent exhaust circulating in there. It's expected lifetime will be measured in tenths of a second IMHO before significant failure begins. I would anticipate the bottom of the rocket may not even be there by the end of the burn.
For what convection can do, here is an example. I used to have a 6" diameter roughly 80# rocket that I flew a number of times. On a research 114mm O for supersonic flights, the bottom of the thrust ring was fine. On an 88mm research N flying transonic, the diameter ratio was such that a convection cell established at the base of the rocket when it was moving. Now the motor tube stuck a half inch or an inch (don't recall) past the thrust plate. The thrust plate was an inch of plywood with a carrier plate of a half inch bolted onto it. That carrier plate was birch aircraft plywood, epoxy coated, and heavily painted with decently high temp engine paint. In about 5 seconds, the exhaust plume convection, which isn't naked eye visible, and the heat radiation, charred off 1/16" of the plywood from the bottom of the rocket. It did this every time I burned an 88mm N. Now these motors are a little hotter than your typical commercial motor, but still... That was a flat surface that the motor tube stuck through, and there was no tubing lip or anything like that at the base of the rocket.
In your configuration, imagine the exhaust plume confined inside your rocket. That's what you've done.
Were I the RSO, I wouldn't allow it to be lit. It will come off the pad much slower than you expect due to thrust loss. It will quickly burn out the bottom of the rocket removing the fins. In the process of burning out the bottom of the rocket there will be a time period with potentially severe asymmetric thrust. The rocket will veer off course. THEN it will have full thrust, and likely be unstable to boot. Sorry.
Gerald