Clayton,
Do not melt your lead in the pot you are going to use to pour your jigs, because you will get a lot of crud on top. Melt your old lead in another pot, skim off the crud and garbage, then flux your lead and pour it into small ingots, so you can put clean lead into your pouring pot. Also remember to be very, very careful if this is your first time with lead. It is extremely hot and there are no second chances if you get burned. Do not have any kids or pets around and make sure you don't get interrupted. Don't smoke or drink any liquid when you pour. Water and lead do not mix. Again think on the side of extreme safety. There is so much to cover here that I cannot even begin to tell you. Is it worth it for 40 lbs of lead ? In my personal opinion, No. By the time you buy everything and clean the lead of all the impurities, it is cheaper to buy your jigs or whatever you want. If you are going to go into selling them, well that is another story. I get lead for $1 per lb I strongly recommend you go to the wire bait section on tackleunderground.com and read up as much as you can on pouring jigs. It also has a thread on there regarding fluxing and safety. I am mentioning this not to promote their site, but to make sure you don't get severely burned. Once you get burned, it will take months to heal, and then the scar. I'm not trying to scare you but this is serious stuff. Make sure you wear, safety glasses, face shield, long pants, long sleeve shirt preferably quilted, socks, shoes and gloves.
Moderators Note: Site mentioned is not my site and I did not mention it here for promotional use. Site mentioned is so OP knows all the ins and out of pouring lead and all safety factors needed .