I've used Ultra Copper without fail. The last time the intake was off was in '00 and no leaks

 

My 381 went together in 2001 with Ultra Copper - no problems.

You don't have to use studs as guides for the manifold, you can use wooden dowels.

 

I also used the wooden dowels for studs and it worked great. Used the Ultra Copper and have had no issues either.

 

Heres a trick I learned from an old school GM Technician. Take a center punch and put dimples on both ends of the block where you apply your RTV. The dimples in the metal will give the RTV something to hold onto after the Intake has been set. Ive been using this trick on every motor Ive ever torn down and havent had a leak yet.

The best RTV I have found is MOPAR black RTV for Engine repair and MOPAR Grey RTV for transmission and aluminum parts. It's only a couple bucks more a tube but well worth it.