IMO there is no reason this car shouldn't be in the high teens on the highway running 60-65mph.
360, xe268H, headers, 2.94 gears, 2500 converter with a 750DP got 20 MPG.
He's got the same basic final drive.
IF you are low in the rpm range, jetting may not change your mileage. It may require IFR and IAB changes to get it to come around if still on the transition circuit at cruise.
One of the guys on moparts did a great multi-page write up on his tuning with a wide band. Subtle changes like float level made a difference.