FE intake manifold fitment can be a pain in the booty. You have the intake flanges to worry about, the ends, and then because the intake makes up part of the valve cover flange, you have to worry about that too. The fact that the intake gaskets are submerged in the
oil doesn't help.
A good quality gasket is always necessary, something like a Cometic, Edelbrock, Mr. Gasket, etc., and it needs to be used after the intake flanges are deemed flat, straight, and at a parallel angle to the head.
I glue the gaskets down to the head, and then use a little sealant around the ports before I set the intake on. You have to also pay attention to how centered the distributor is in the hole, because if it's off, it could cause the distributor to be hard to turn, or could cause a leak on one side.
The guy working on your engine is correct, you don't want excessive lash. It's really not a valve spring issue, but it's just harder on all parts involved, especially when the cam is not designed for it. And as noted, more lash will effectively make the cam seem smaller, just because actual duration from where the valve is actually moving is cut down.
What's in your favor is that with every cam, you usually have a little window of lash that you can work with, and there are many times that I don't follow the cam grinder's spec. Your .014-.018" cold spec should be ok, IMO.