will higher compression help gas mileage?

ive got a 71 mustang convertible with a rebuilt 302 with performer cam and intake, c4  tranny, 3.23 rear gear.i was going for mostly close to stock rebuild and was told the 1970 351w 4bbl heads with bigger ports an valves word work fine with combo. the heads are also about 5cc bigger than the 302 heads. now i find i have to run a lot of timing or its real lazy at low rpm. 16 initial 34 by 2500rpm.the best mileage i get is 11-12 mpg.any ideas opinions? would i be better off swapping back to 302 heads. do you think compression is the culprit or the bigger ports/valves?  

what you are describing sounds like a low velocity problem caused by the bigger ports...the 5cc difference in chamber volume won't make a big difference in the compression ratio but the larger ports will slow down the intake airflow until the motor spins fast enough to make use of them  
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It's the bigger valves and ports.I have a 70 Ranchero with a 351 Cleveland with 4v heads. I've learned a great deal about big valves and huge ports.Used to have the same results.Stumbles off the lineand comes alive like an animal at 2200rpms all the way to 7000rpms.The cheaper way out for me was to drop down to the 2v heads.I went the expensive route.Stroked it,gave it a BIG roller cam,ported the heads,match ported the intake.Results to 419cuin.608hp.For you,if you have the money get the new Edelbrock heads for your 302.They perform very well.Get rid of the stumble off the line and save you money at the pump.First time I built my Cleveland,I pulled 525hp at 5600rpms.With the FMX trans.,same 323 rearend gears on the freeway 9 miles a gallon.That's 3200rpms around 70-73 mph.The stock 302 heads are very restrictive.Cost alot of money to get the to perform.I know this for a fact,been down that road.Spent $1600 on a set.Bigger SS valves,bronze guides,harded seats,pocket ported,ect.New Edelbrocks heads under a $1000.If you can't afford that,take your stock heads to a good shop.Put new SS valves,harded seats and a 3angle valve job.Spend the extra $100 to $150 for match porting intake to heads.Believe me it's worth it.  

thanks guys thats pretty much what i figured, since pulls strong when you start revving it.the edelbrock heads were  my alternate plan when i built the motor.  i had the 351w heads from a past project and only needed springs so i cheaped out when my machinist said they would work. they work great when you twist it up. oh well! thanks again  

The stock 302/351 heads def need port work to perform.