As for the new battery not cranking fast enough, I'd suspect either a faulty battery or poor battery connections. I'd go back and clean the battery terminals and connections real well, and try that. Make sure the battery is charged. When you put the booster on it, you, to some extent, bypass the harness to battery post connection. If it still won't crank after you've cleaned the terminal connections on a charged battery, go get another battery, that one has a problem.
With all the batteries I've bought, (dozens) I generally buy whatever they have on hand, sometimes at Walmart's "sale" display, sometimes at the local NAPA. They all start the tractors easily regardless of CCA. The difference between them is longevity. Cheaper batteries seem to only last 3-4 years while the better ones last 5-6 years.
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