cracks in the shower pan are not good and normally will not repair easily with the weight and all. I would consider a new kit
Secondly, no matter how tight you turn the handles, if the seats and stem ending is burred or knicked, it will never stop leaking. Get the right tools and remove the valve seat then take it in and get the correct size replacement, then check the stem end for burs, replace all at once. Not a big cost at all.