I was alway's told "do it right the first time"
I've alway's went by that.
Buy a heatercore from 50resto.com that last forever and do it right the first time.
It's not that bad dropping the dash, it's just time consuming and require's patience.
DO NOT CUT THE FIREWALL, you WILL regret it.
I can understand cutting a spot in the trunk to do a fuel pump but not a heater core.