Heating, venting issues

Hi, I am starting my first medical marijuana grow. I have been testing my grow space for a week and need another opinion. I have two 600 watt hid mh lights, and a great carbon filter/ fan combo. The space is a framed in 6x6 room in garage. Growing 6 plants, white widow and thc bomb. Problem: I can keep temp at 81 during light cycle, but due to very cold weather, temps dip in room to 58-62. I have a very small heating fan and a old radiator heater. I could manually keep it warmer by turning fan to max heating, and the same with heater for night cycle, to get it warm enough, I need to have 412 cfm fan turned off also. Should I manually adjust heat everyday while cold and not vent for 6 hours, or let the room get low 60's? I also have dehumidifier that keeps humidity down to 30%. Please advise, and thanks ahead of time.

It's fine in the low 60s, you need a 10-15* degree drop when the lights go out. The plant will stop growing around 55*

Thank you so much