heater core or matrix

i have a 1990 525i that im replacing the engine on now i had a mysterious loss of coolant and the other night it just overheated and wouldnt bleed out. it looked like i was driving a rocket ship. New headgasket and everything leads me to believe i have a cracked block considering the mysterious loss of coolant. One thing i noticed on this car though is that sometimes the heat would smell of coolant. and i would have puddles on the driveway if left overnight. All 3 coolant hozes are new and has new waterpump good headgasket install etc. Also about 6 months ago i replaced the heater valve because it was leaking and melting my fuse box. Lastly the rear vents had no heat what so ever it came out cold even if the heat was on full hot.and i had heat in the front. Should i replace the heater core or matrix while i have the engine out? or do you think it was just a cracked block.

Thanks
Jason