When I pulled the intake recently to change the cam I noticed that I had the rubber end seals, I didn't even remember installing them, anyway, I left them there but applied a liberal amount of RTV to them upon reassembly. No leaks before or after.
Used 5 8 years ago the end gaskets together with red high temp RTV and no leaks. Last spring I installed the tripower setup, again used the rubber seals with red RTV top and bottom of them and is still OK.
I trial fit the intake first with only the end seals to assure that they are not squesed too much preventing the intake from siting down enough.
Using only RTV on such a high gap ?? I think definately OK if the block is decked.
Let's see ... this isn't easy to explain but here's what I did: first I installed the intake dry with thick shims instead of the gaskets. I tried to line up the boltholes first. I determined that I need a .120" thick intake gasket to line up the holes in the intake with the threaded holes in the heads.
I measured the gap at the front and rear of the intake and it was about as big as the uncompressed rubber gasket. It would not have sealed with the rubber strips alone.
I used RTV and let it cure for about 3 hours, then installed the intake with it's gaskets but did not tighten the bolts yet, I waited over night and then torqued the bolts.... no leaks, it's been over a year, so far so good...