Scs1200 is an adhesive silicone, acetyl cure, extremely strong, very quick set. While it is no doubt a great choice for large glass tank construction it may not be the ideal choice for a novice to reseal a large tank.
For a reseal job, the tensile strength of the silicone may be the least important quality. You want a sealant that you will be able to work into every micro-nook and cranny of the seam--and not have to hurry doing it. Opt for a much more workable silicone sealant like GE silicone 1, DOW 732, 995, 999, or 795, momentive 103 or 108, or the ASI option above which I believe is just a rebranded version of one of those. Ive also been seeing a new brand called kraken that is guaranteed for use on aquariums that looks promising, but I haven't tried yet.
Most important thing with the reseal is that you completely remove every trace, smudge, sliver of old silicone before you lay in the new bead. It's amazing how one small spot of weak adhesion ruins an entire reseal job.