Just a thought here - did your shop square the block? Standard height is 9.025 and it's not uncommon for a block to be a titch out of square. You can take a couple thousands off with out much problem, but if they took the block below 9.020 and didn't shave the intake manifold it will be really difficult to get a good seal.

That said, if the block hasn't been cut or if it has and the manifold milled to match, I'd remove the intake and proceed as follows:

Lay a few rags in the lifter galley to catch fluids and woolie goobers. Clean the head and the engine block with acetone. (Goes without saying - please put on some safety goggles before using the acetone - I'd feel horrible if you got a splash in the eye and you'll really feel horrible!!) Remove the rags and clean any obvious little bits of gasket. Use some Gasgacinch or similar sealant on the heads and on head side only (none on the manifold side) of the gaskets – this is important – let the Gasgacinch dry until it’s no longer tacky and then set the gaskets on the heads. Do not use the rubber end seals that come with the gasket kit, rather a good RTV silicone sealer instead. I use black, but I’m not sure it matters – my dad swore by the blue RTV and he built a ton of “leak-free” engines. Put a good bead on each block end seal surface – make sure to overlap the intake gasket about ¼ inch at the four corners. I let this sit for a “full-cup-of-coffee” before setting the manifold.

Follow the instructions with your manifold for the sequence of tightening. I put a small blob of silicone on the eight inner bolts to prevent oil leaks (irritating if you forget and get a bit of oil pooling on your nice new manifold). Torque value for pre 1986 is 20-25 foot pounds. Vortec heads usually are good at 10-15 foot pounds – resist the temptation to over-tighten as it will squish the silicone out and you’ll be right back to square one.

Change your oil and you’re good to go!

Have fun,
Glenn