273 exhaust manifold gasket

I can't imagine how coolant would enter the exhaust due to those studs leaking. They will leak to the outside of the block.
The original exhaust manifold gaskets for a 273 was also a sheet metal shield protecting the plug wires.
With todays heat resistant wires you really dont need those knickle slicing shields and I dont believe thay are reproduced any way. Common 318 gaskets will work.
Removing those studs ( 2 forward most and 2 rear most in water jackets ) can be tricky. They break before backing out very offen. If you can get them out the next step is to get the studs and the holes clean and dry so a hi-temp silicone ( Permatex ) sealant will bond and stop the leak. Good luck