70 is a bit low. If it's having a tough time turning with that compression my guess would be either 1: Wrong size cables. 6v cables should be at least 1 gauge, 0 would be better. I've read that factory was 1 gauge. 2: Weak battery. 3: Dirty connections on either end of both battery cables. 4: Weak starter. Any one of those conditions will make it turn over slow. Most likely you'll have more than one of them, most do.
#2 Make sure your timing isn't too advanced. If it's firing too early it'll make it crank slow.
#3 Are you getting spark?
Most slow cranking issues are solved by some combination of #1.
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