I have a ported plenum, slp runners, and tpis big mouth intake.

I was trying to mill out the gaskets but they are now so thin. Can i just use silicone instead?

 

lol, silicone will work for anything. No guarantee that it will work best though.
If your gasket mating surface is thin, then why not build up the surface and reform it?
Even silicone needs surface area to hold a seal.

 

He means the gasket ( w/steel shim inside) itself is too thin to machine out to size of ported runner

 

yeah that is what i mean...

 

Ok you mean the stock runner gaskets. Yeah they are very limited.
You can grab gasket material from the parts store or hardware and cut out your own. I have to do this quite often so I keep a couple of rolls around.

RTV does pretty good with filling in gaps. Just give it a good 24 hours to set up before running the engine.
Permatex high-temp is good quality stuff.