383 won't start. It cranks, I have spark & fuel - could it be the battery

And once the cranking speed is slowing down after considerable cranking, you are done...until you wait a long time, charge the battery again, etc...remove and dry plugs....
But while the battery is charging, get a 5 gallon bucket, fill it with HOT water.
Spend 2 minutes pouring that hot water on the HEADS of the engine.
If it's below zero, you'd need 3 buckets of hot water.....
By this time the battery has enough to try again.
Be sure to give time for the gas to vaporize, or just use ether alone..... if you have a paper element filter, give it a 15 sec starter fluid soaking, put it back in the housing, make sure the choke is off, and see if it fires on clean ether.

Slow cranking won't do...
The main thing is to get the heads warm.
Gotta hope your valley pan seal is good.......