Domi: Sounds like you need to do two things. First, have you matched the manifold-to-head intake ports? If you, you should. Before I go any further, it sounds as though the heads have been milled or the block decked, which wouldn't moved the heads down. This will not only misalign the cylinder-head runners with the intake ports, it will move the manifold down and closer to the block. This may be why the gaskets are splitting . . . they are getting squished. The cure is to have your manifold milled, sides and bottom. Read on. Check runner to port alignment first.
If you find that port alignment is okay, do as what has been suggested and run a bead of RTV silicone across both ends of the block. Be careful not to apply too much as it will squeeze out, both inside and out. Chunks of silicone falling into the lifter valley will clog the
oil pickup. Have your manifold ready to install. Set it on the engine before the silicone skins. Run the bolts in finger tight, then torque them in stages and in sequence. This should cure the problem.