Designate left and right heads, scribe each chamber through the cylinder block with the cyl bore, if it's a later 427 be sure to catch the valve reliefs in the cyl walls,  then open up the chambers to your bores and pay attention to the walls in the chambers that are right up beside the In & Ex valves besides relieving them you can do a slight undercut in the wall by the seats. This undercut is said to allow the flow to start earlier and gain momentum at low valve lift.  If you note in the stock ED heads the valves are partially blinded by the chamber walls until the valves are seriously open.  Big valve swaps just make the problem worse.  Someone here just stated that Ed Heads are not true M/Rs.  I have a feeling they are a bit narrow on the intakes -  do some searching for different Int gaskets and base your work from there. The Ed Heads obviously have a dam good designs in the ports as delivered, they were designed decades after FORD and with Eds offices full of port gurus.  Keep in mind you want to enlarge but still maintain the stock profiles  -- not being creative and hogging the hell out of them here or there. Obviously the valve guide support areas are a major impediment that you can spend plenty of time on.

Remember the Ed Heads were made to match up with 390 bores the bigger the bore the more need for relief. 

JMHO

Jay was discussing 69/70 SHELBY CLONES with a friend and the discussion got to how much "SHELBY" costs.  He throught about $3,000 for the 69/70 (SHELBY) body parts obviously F-Fenders, Hood, Qtr Enxtensions, Valences, Lights/Moldings Misc (SHELBY) parts like supports, chrome, grille$, Emblem$, Decal$ etc  and that does not even address the interior trim/parts that obviously will be somewhat missing in your car.  Jay BALL PARK FIGURE what the "SHELBY" exterior parts/pannels above totals up to?... I have a feeling the $3,K my friend tossed out might not go to far when it comes to SHELBY-LAND... I'm going with errr errrr $8K....

             Thanks
     

Despite the BOSS 302s cool specs I don't think there has ever been so much disincentive as right now to not buy a new Boss 302 or any other Mustang.  The  disincentive is one word as mentioned in the above reply in odering a new truck: ECOBOOST.
         To me the ECOBOOST motor is obviously years ahead of anything else offering a small engine with big engine power across the band, proven by FORD durability and fuel efficiency.  Obviously all FORD motors will soon follow in the EBs path (the GTs new 5.0 already has the bosses in the head castings for direct injection).   You just know unless the "economy" shit hits the fan that in the next few years BOSS and other MUSTANGs will be ECOBOOST based with that advantage.
         

Decades ago I visited Torque Man Enterprises in Glennville Ga. They were leaders in monster 385 series FORDs esp for truck pulls. One of their "tricks" was brazing or welding on what looked like 1 to 1-1/4" Deck Plates to the top of the cylinders, then boring and slieving machining the works so that it had enormous Cubic Inches. I happened to call after a week-end just as a blown Puller Motor was brought in. They were studying it in shock. It had blown the 4-bolt lower end so hard out the bottom of the block that the mains with caps and block sections with studs were all still intact around the crank. It ripped the entire bottom off the motor the carnage was out of this world.. Just keep in mind that the crank needs to be supported.