You tore down the top-end of the engine and drilled out the block to match the cylinder heads? Your 402 engine is a Mark IV big-block, which has a specific deck surface design to transfer water up into the cylinder heads. With this design, GM was able to make the cylinder heads and head gaskets interchangeable from the left to the right side of the engine. The coolant passages in the head gasket controls the amount of water flow from the block into the cylinder heads. If you notice, the water comes into the block from the water pump and is forced to travel to the rear of the block before most of the water is allowed to travel up into the cylinder heads. If you modify these holes in the head gaskets or drill new holes in the deck surface to modify this water flow, you can bypass cool water from the front of the engine right up into the heads. This would send this cool water out of the engine into the radiator before it’s done absorbing heat. The front of the block on a Mark IV big-block’s deck surface is solid and has no water transfer holes like it does in the rear; this prevents the water from going right into the cylinder heads from the water pump.
As I said, GM used the block to control the water on the Mark IVs. On the ’91-and-later Gen V and VI big-blocks, the front of the block does have the water passages, and they used the head gaskets to block the transfer of water in the front of the block, however, you cannot use Gen V or VI head gaskets on a Mark IV. The gaskets will not seal several water passages at the top of the deck and will leak water in the lifter valley of the engine.
The last year that GM built a big-block Monte Carlo was in 1975. We take it you have outfitted your ’76 with a big-block radiator, fan, and shroud to cool the beast? The fan shroud is very important on the Monte Carlos because of the distance from the water pump to the radiator. Also, make sure you have the factory air dam that was attached to the bottom of the core support to prevent hot air from the engine compartment migrating back into the front of the radiator and recirculating hot air back through the radiator.
Finally, if you are having heating issues at speed on the highway, and not at idle around town, we’d look for a lean condition, or not enough spark advance at cruising speeds. Make sure you have a working vacuum advance system on your distributor. At part-throttle cruise you would want to see 40-44 degrees of spark advance putting down the road. This will help with fuel economy and engine temps.
Hopefully, you haven’t drilled passages in the deck to circumvent the flow logic of the cooling system. Make sure you’re running all the factory shrouding and the proper clutch fan to engage when the engine temps climb. Hope these tips get you cooled off.