Only seal needed is the foam gasket you seen.
The gasket when torqued properly should be roughly 1/2" thick.
This is what it looks like with a/c removed.
There is sealant there only because a vent was removed prior to installing the a/c.
The sealant was 90% removed before a/c install.
Torque on the a/c mounting bolts can be checked . 40 to 50 inch lbs.
2008 2500hd Duramax
2015 Columbus 320RS
2016 nights camped "0"
2015 nights camped "34"