This is exactly what I was planning
Luckily I am aware, been in the business for a while
I think I'll be fine going at it with a soldering iron moving around on top of the heatsink. It's easy to see when the solder melts. I can control the heat output of my iron to the degree between 90-310°C.
I think it should work, most of the solder is rated at 180°C and below.
I buy the lead-free shit for health reasons mostly, solder smoke is bad enough as it is.