The older 351 heads 69-74 have smaller combustion chambers(for more compression) and bigger valves than the newer style 351 heads.They would be great as a bolt on head but if you have to rebuild the heads anyway (which is likely with 35 year old heads) you might be better sticking with the heads you have.You can have the larger 1.84/1.54 valves installed in place of your smaller 1.78/1.45 sized valves .I'm not sure on the combustion chamber size of your E5 heads but the old 351 heads are spec'd at 60.4 ccs.You might want to talk to a machine shop and find out which is the best way to go.I've seen lots of these heads at the Barrie Automotive Flea market for $200-300 bucks but they would still need rebuilding.