The machine shop where I ran the connecting rod reconditioning department had a BIG belt sander- - - -like 18" wide and 4 feet long. It was perfect for truing up gasket surfaces- - - - -intake and exhaust manifolds- - - -flathead six cylinder heads, and any other surfaces that had to be straight and true across a long, wide area. Just use a carbide burr in a die grinder to enlarge those exhaust manifold end holes. The ends of the manifold need to be able to creep a little in the fore/aft dimension for heat expansion purposes.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!