68-70 Manifolds on 72 340?

Are you asking for physical fit or whether the engine will not lose anything going from the Doug’s to the 340 HiPo’s?

Sounds like you may still be in the headers category to not choke it down. What’s the specs on the cam?

There was a really cool article in Hotrod where Vic Edelbrock got an early 340 and started doing testing and development on the ‘new’ 340. They started out pretty basic and added a part and then did a dyno pull. It was the first time the LD340 had ever been in print.
Mopar Manipulatin’ , HRM August ‘69