harman boiler mod

[grader]

i posted this in the boiler section but got no response . i just installed a boiler that is the same as a harman sf360. i would like to do some mods to it next summer as it is now in use for this heating season. i would like to add some secondary air intake up near the top of the firebox. there is 4 or 5 water pipes running front to back in the boiler and i figure i can bring in a pipe from the bottom back of the firebox up the back wall, install some tees, and run 2 or 3 pipes between the water pipes. under these pipes i will weld in a piece of angle iron on each side to hold a metal plate or shelf. this plate will be the same width as the firebox, but about 6 inches shorter than the length. when i open the door the plate can be moved towards the door to allow the smoke to exit strait up the back. when i close the door i can slide the plate back forcing the smoke to the front and over all the pipes. air control for the secondary air will be through a normally closed valve powered of the primary air damper. now some questions. what diameter pipe should i use? what kind of pipe will stand up to the heat? should the steel plate have some insulatave refractory placed on the top to better reflect heat. the volume of the firebox is about 7.75sq. ft. i know some here have done similar mods to wood stoves, any comments or advice about these mods are welcome.