Can a intake manifold leak on a 91 305 let coolant into the exhaust?

The intake could, but it is real rare because of the distance between the water jacket and the intake runner on that year engine. Most of the time that a intake is leaking coolant it will run into the oil pan and the engine oil will turn white.

Your problem if the engine is steaming white out of the exhaust and over heating is a Bad head gasket or crack in the intake or exhaust valve chamber in the head.

Need to magnaflux the heads for cracks before reinstalling them.

Check them with a stright edge to make sure there not warped too.

Sounds like it was over heated. GOOD LUCK NEUTZ.