Manifold Bolt/Stud Replacement and Exhaust Leak

I am new to this forum and new to owning an older tractor so please bear with me. I bought a 1970 Ford 2000 3-cyclinder in September that I have been working on. Bought it from a local guy and it runs just fine, including the brush hog that came with it.

After doing some maintenance including cleaning, some rust removal, and fluid exchange (transmission, oil, hydraulics) I decided to take on replacing the exhaust manifold, which is the most rusted and corroded part of the tractor. I bought a new manifold and gasket off Yesterdays Tractor, and picked up a new muffler. When I took the old manifold off, the bolts were pretty rusty so I soaked in rust remover. That worked pretty well. The main bolt/stud(what is it called by the way?) had a square head that was pretty rounded, but I was able to remove without breaking. So I cleaned everything and put the new gasket and manifold back on, tightened all the bolts. And gave it a start. Two things happened after warming up: the exhaust started leaking from the manifold and exhaust smoke (while going up the muffler at lower throttle) started circling around the muffler at higher throttle. What did I do incorrectly? It worked fine before I started disassembling things apart and putting new parts on. And another question: I cant find that manifold bolt/stud anywhere online. It's 5 inches long and a picture is attached.

You have to make a few posts before you can add a pic.

Thanks. I'll try and add a photo after a few posts. To describe, its 5 inches long, about an inch of thread, and a square head. I goes in the center of the exhaust manifold at the top. The other 5 bolts are much shorter with hex heads

I don't know anything about the 3 cylinder models. did you look at YT parts for that bolt?

I did look on YT as well as Steiner but didn't find anything in similar length. No markings on it either

I was able to upload a picture.

It's a common 5" long 3/8" bolt you can pick up at a hardware store
Originally it would have had a hex head not square
NH part # 300960s36

Thanks so much for the clarification and the OEM part #. Ill see if I can find at hardware store. Now to the exhaust leak question and the exhaust circling the muffler, both at higher RPM, any thoughts? Do I just need to check the gasket and make sure I torqued the manifold bolts enough?

As you were cleaning the mating surface of the head to a mirror smooth surface before installing the new manifold did you come across any low places?