I'd bring it back to the mechanic? If you want to do it yourself, use RTV. First fit the manifold dry, observe the gap at the front and back. Remove the manifold and make a bead along the wall just a bit higher than the gap you observed during the dry install.

 

The secret is to follow the instructions in the Chassis Service Manual for installing the stock metal intake gasket. Hopefully the mechanic did not try to reuse the original - they cannot be reused. RTV is required and the four locating features in the corner of the gasket must be properly fitted into the holes in the heads before installing the intake. This must be dry-fitted to ensure proper fitment prior to using the RTV. Also, the 8,000 lb iron intake is nearly impossible to manually install while leaning over the fender without screwing up the position of the gasket. My aging back has forced me to use an engine hoist these days. Using RTV instead of the rubber end gaskets is common, but one needs to be sure the beads are tall enough to seal. I use Permatex Ultra Black in the aerosol can - it's the best RTV I've found.