Sometimes when I start my 89 Firebird 305 TBI it blows out a large cloud of white smoke for about 5 seconds. Seems to happen whether cold or warm, and not consistently. Other times it starts fine with maybe a very small puff of white smoke and some visible smoke coming out of both tail pipes. I can also smell fuel once in awhile.
Runs very well at normal driving and on interstate (up to 80 mph).

Specs:
305 TBI with 51,000 original miles. Bought it 2 months ago with 48K.
Oil about 1-2 quarts low (checked it today). Oil looks normal.
Coolant in reservoir brown-ish green color, coolant in radiator looks normal.
Recently had tune up -- plugs, wires, distributor, cap, rotor, ignition coil replaced; fuel system induction. O2 sensor replaced (had a code Service Engine Light Code 44 but the O2 sensor fixed it).
New Magnaflow exhaust system installed about two weeks ago.
Idles at 700-800 rpm in Park; 500 rpm in neutral.
No visible leaks on engine.
Hoping it's not something serious like blown head gasket. Any ideas would be greatly appreciated. Thanks.

 

My guess is the valve seals are leaking

 

x2 or you have some residual water pooling in the system somewhere blowing off when you start it.