These guys are telling it straight, your speedometer gear will be off by a percentage of your speed. Look into buying another gear with one more tooth. There are speedometer companies that sell a 'box' that they can dial-in. Your speedometer will be right on the nose afterward. Your tires may be shorter than the originals. Get with a good tire company and see if they have tires that match your original tire heights.
Exhaust manifold bolts are 3/8" diameter. Buy stainless steel socket-head cap screws and lock washers. Going to a harder bolt will not help because your exhaust manifolds get cherry red hot. That anneals carbon steel, drawing the carbon out which makes the bolts soft again. 18-8 stainless does not have a carbon case harden. If you cannot find bolts in your area, check eBay.
Exhaust manifolds don't cost a hundred bucks to machine, here in Detroit. (I just bought a pair of FE manifolds for $100.) These manifolds had NO gasket when your car left the factory. Flat manifolds need no gaskets. When you tighten exhaust manifold bolts, they feel solid, not spongy. - Dave