273 with 340 exhaust manifolds

The early 273 drivers side manifold had a near 90 degree crook cast in. Quoting what someone else once said "Like trying to poop through a straw". I suppose it was engineered adequate for the stock 273. They only changed the Y pipe diameter for the commando with 4 brl and more. Anyway...
If you're upgrading the air in, you probably should upgrade air out one way or another.
From what I have read... Some have successfully installed Dakota manifolds with numbers ending 618/619. Others have failed too. I don't know if their heads were different ( 302s maybe) or different Dakota manifolds, or just what led to their failure. Those Dakota manifolds run very close to firewall thus they heat up the sheet metal.