400W HPS short cab

I'm trying to get together all the elements and designs for this cab. I'm going to try to make it 2.5'W x 2.5'D x 4.5'H (maybe 4'H). For it to be so short I'm going to make a cool tube. I just bought a pyrex bake-a-round from eBay for ~$25 with shipping, and a 400W HPS with a batwing reflector from Inside Sun for ~$120 with shipping. I'll make the tube open at one end, so the main exhaust for the room will be from the cool tube. I'm gonna get two inline duct booster fans - 8" 210CFM - $34 each from Home Depot. One will be for intake.
I still have to wire the area up, as it is in an area under the house (thus the height restrictions). If the temps stay pretty cool down there, does it sound like my ventilation system will keep the temp reasonable?

1 210CFM intake fan (possibly on 15min interval timer, to get better environment control)
1 210 CFM diy open ended cool tube exhaust.
400W HPS

I guess I will just try this out with light and no plants, and see what the temps and humidity get to. If it stays too dry then I will dedicate an intake and exhaust cool tube sealed off from the rest of the room, and vent the room separately. In a smaller - not very well ventilated, or air sealed at all - grow cab in another part of the basement I haven't seen humidity rise above 20% since it started getting cold and dry outside.