In the past I've had good manifold sealing success with just smearing grease (no particular type, but I've probably used bearing grease as often as not) on both sides of the gaskets (after cutting to size with a die grinder/dremel if required, as mentioned above) and running the motor like that. the grease sort of burns its way toward a better seal. You will get some smoking (both in the engine bay, and out the pipe) upon initial start/running, and I'd suggest running the motor for an hour or two before really pushing it (just to make sure there's no grease left to get drawn into and contaminate the intake charge)

On the dremel topic - I'd strongly suggest checking the header flanges vs the port exits (use the gasket as a template if necessary, or look for carbon deposits on the flange if it's already been run, which might indicate an overlap/misalignment. Anyhoo, opening the head side of the flanges so there's about a 1mm larger step from exhaust port exit to header flange entrance, not only will it contour the flow - funnel it to where it needs to go, but the small step acts as an anti-reversion step, helping prevent exhaust gas reversion. This tiny bit of extra work can gain the odd hp or two, but it'll also potentially clean up the idle and even get a race motor to 'come up on the cam' a few hundred rpm sooner vs unchecked/tweaked header flanges. Since it only costs the odd hour or two of time, it's a good option.

I recently found pics of a set of headers on a mates car from years back. The flanges were big enough and shaped so that their positioning couldn't easily be moved, but the holes to match the exhaust ports were not lined up within the flange. If the front one was aligned, the end one was close to 1/4 of the port fully blocked off! It obviously took a bit of work (I had to weld up the outside of them, because they were so far out of whack, there wasn't enough steel left to grind them out to suit the port location without breaking through the sides. It's not a datto exhaust header, but the pic of the carbon buildup on the flanges due to the misalignment on the worst ones is significant and maybe of some interest. If anyone wants, I'll include a pic in this thread.