The clock will drain the battery over time. So yes, disconnect the ground.

As for brands, I've just bought blemishes for my car the last 2 times. $35, the one before the one I'm on now lasted a while even with being 100% drained 2-3 times from sitting a while. As for size, I THINK I had a group 49, but it's been years - other members with a 4.5 can say what is in theirs. The tray is massive & I took advantage of that fact.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k