I have kept gizzard shad alive for days in the summer and weeks in the winter. Water quality is key,amonia will kill shad quicker than anything. You need a good tank with aeration and circulation, and a filter system to remove all of the solid waste. I use zeolite for amonia removal, it can be purchased in most pet supply stores that sell fish.
After you catch the shad and get them in the tank they will purge. During this time you will need to keep the filter material cleaned out and maybe do a partial water change. Once they have been in the tank for 20-30 min they will have cleaned themselves out and it is easier to keep the water clean. It is important to use salt in your water. I use the salt sold at Lowes that comes in the blue bag, 1-2 cups to every 20 gal of water will be sufficient. Never change the temp of the water in the tank more than 5-7 degrees at a time it will shock the bait.
It is work to keep good lively bait but it can be done.
No need to feed them they can survive for weeks without eating.
I hope this helps if you need any more info pm and I will go into more detail.
Big Bait catches Big Fish.
If you think your bait is too big, then it is just right.