Or, buy an Exhaust Manifold Gasket, and use it as, well, a Template Template. Lay your holes out with its holes.
This is what RockAuto describes as a Perforated Steel Gasket, from Victor, Part MS15499. I call them Stamped Steel. They're on the idea of a spark plug gasket/washer, forming their seal by crushing under torque. Chevrolet used a lot of them for Head Gaskets on Small Block V8's also.
I actually DID use such gaskets in our 1983 E350/460 Class C. A mechanic on another RV forum told me NOT to torque them down hard. I think spec is 45-ft-lb (but verify if you want to use OEM torque on yours!). He said, use Nickel or Copper Never-Seez. Apply to Head, Manifold, Both Sides of Gasket. Torque gradually till torquing gets harder, meaning the steel is crushed, and stop there. Worked out to be about 25-ft-lb. Man's name was Kent, and his idea was to let the head and manifold have an easy job of expanding/contracting differently with heat. That, instead of trying to lock them together and try to force them to change equally. That was Ford's fix. New Manifolds, No Gaskets, New Bolts, Torque the Snot out of it. Around 45-ft-lb, maybe more. Lock'em Together.
I assembled ours with Stainless Cap Screws, Lock Washers, and Flat Washers. If I had to do it again, I'd do Grade 8 STEEL Cap screws, and Stainless Flat Washers. Washers keep the expanding Manifold from hanging up on the bolt heads and trying to slide the bolt heads sideways. Lock washers aren't going to have any "spring" left undeer heat. I'm told Stainless Fasteners get hard and brittle under heat, about impossible to drill if another one breaks. "Harder than a Tax Collector's Heart" was NormK's quote here.
Make sure your fan shrouding is all there, radiator clean, fan clutch good, and exhaust not restricted. I think part of the reason I didn't have any more manifold trouble was converting exhaust to a 3" Flowmaster system.
Oh, Stamped Steel was what Ford used on many 460's, and that Victor is listed as an "OEM Style" gasket. I agree with those who say they would NOT use the Foil/Fiber ones that are also sold for this engine. She'll eat'em up.
Van Noses, whether trucks, RV's, cube vans, or just vans, are hard on exhaust manifolds. They warp them because the van nose doesn't allow the air circulation around the engine that you see in pickups.
Also, let the engine ease back from highway hot coming off the freeway. Let it idle a little before shutting down at the fuel island.
