I aint no car mechanic, but considering the versatility of the Ford engine over the decades, sound like you just blew the heads or piston rings. Basic mechanics says that smoke coming out the exhaust means blown rings, if its coming out the engine compartment, it means more. Exhaust header gaskets blown/worn, or improperly installed (could explain under hood smoke), or even a possibly blown head (I dont know head mech) . Blown rings would explain the loss of power, so would a worn clutch. No compression to drive the pistons etc. Oh yeah, you do have an aftermarket exhaust header, but bet you used the gaskets that came with the kit. I plan on installing a BBK shorty but will reuse the header gaskets due to ford spec gaskets being better (not that has anything to do with loss of power).
Best of luck man.