I don't think there's any danger of you cooking the engine with the 340 intake, as the water ports on that intake should line up with your heads correctly. The only thing you might encounter is vacuum leaks, being that the 340 intake has much larger ports than the heads you're using.
I believe that the 4294 carb only has the NARROW, square bore bolt pattern carb flange. You can drill the carb base for the WIDER pattern that will fit your 340 intake, then you won't need any adapters...or, get a newer carb with the both patterns already there (Edelbrock Performer or Carter Competition AFB )