Simplest is just to add more TO247 PNP transistors and emitter resistors in parallel, as the drop in gain with increased current is going to really ruin your day. That way you run each individual transistor at a safe SOA point ( lower current, the 15A current is as a saturated switch with gain of 2in most cases, not what you want as a voltage regulator) with lower current, and they will also be able to dissipate the heat properly, and as a bonus the gain will be a lit higher per device. giving a better performance.
TO247 over TO3, as the heatsink space is smaller, you only need one hole and insulator ( unless you use a ISOTOP version in which case you only need the hole and thermal compound), they have the same thermal resistance junction to heatsink ( surprise, but comes from not having a steel case but Kovar instead) and are a lot cheaper.
6 for a 30A supply is barely able, giving each a 5A share, but better is to drop it to around 3A using 8, and then for higher current use 8 per 30A block, though there will need to be another transistor then as a driver, your 78xx regulator is only going to be able to supply 0.7A or so of drive at most, even flat out.
Over 20A I would personally recommend using a different topology, using a 723 regulator instead, as you can take this and scale it to a 100A power supply and have it still be stable, and still have current limiting that works. You then also add a power transformer that has multiple voltage taps on it, so you can select a range to minimise differential somehow.