Just verifying what I measured and have read.
For the two bolts that hold the manifolds together, I believe I need a 5/16"-18 4" Flange Bolt and Nut. For the exhaust to manifold bolts I need 1/2"-13 2" plain nut and bolt. All being of at least grade 5, but preferred grade 8 or stainless steel. Does this sound right?

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The Old Goat '83 D150 225-2

AKA Ice Titan

There are 3 bolts (or 2 bolts and 1 stud) that hold the 2 manifolds together.
There are no 1/2-13 unc bolts, the exhaust header pipe to exhaust manifold flange use 7/16 - 14 unc nut & bolts.
DD

Ahh, 7/16. I was using what was left of the old broke off bolt and nut measuring with a ruler to get my sizes. Are my hardware grades correct?

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The Old Goat '83 D150 225-2

AKA Ice Titan

I'm not sure about the expansion properties of grade 8 bolts. I think grade 5 would work fine. Stainless bolts might back out on thier own due to heating and cooling. But there is the chance that I'm completely wrong too. If it was mine I would use grade 5 and plenty of antisieze

I would use grade 8. Stainless bolts are often not as strong as grade 5 unless they are a special bolt. The generic 18-8 stainless bolt is about half the strength of a grade 8. There are high strength stainless bolts but they are usually 400 series material and might be hard to find in the right size. Be careful about over torquing. Many parts that involve the exhaust will expand more than the bolt and will over stretch the bolt and then cause a leak if the bolts are too tight.