I have never seen nor have had a Ford N-Tractor exhaust manifold crack. They will however, blow out right at #4 cylinder port -where the pipe connects, nearest the operator. That area gets the hottest and will burn out the gasket there then start to wear a channel in the cast iron metal itself. If left too long it will start to wear into the block and you don't want that. It's a lot cheaper to replace a manifold than a block. The other issue is that the exhaust manifold uses brass hex nuts to fasten it to the block with and sold in kits -try Just8Ns. They get torqued to 27 ft-lbs. Assemble new manifold with two new gaskets -no sealant required - and torque to specs. After running engine until hot for a good 1/2-1 hour, shut down, let cool down then re-torque. Studs are made with one end having a coarse thread -this end screws into the cast iron block, and the other end having a fine thread -this end accepts the brass nut using a steel lockwasher. If you are using steel nuts, you may be cracking the block that way with too much torque, and as mentioned, whacking off the pipe numerous times will only cause stress on the manifold. I can't comment on the new manifolds -last one I bought was made in India and has been OK. I'd have to perform a Brinnell Hardness Test on the actual metal to determine it's effectiveness. Cheena made items have a poor reputation no matter what the product is. Many manifold kits I have looked at have the new manifold, and the three gaskets -two for the exhaust ports, and one for the carb. Be sure to scrape off all the old gasket material from the block before assembling. Also, soak the carb gasket in gas prior to final assembly. I'd try to buy a new one that was made in India or Brazil for a better quality.
Tim 'PloughNman' Daley(MI)