Ok heres the problems I see based on what you are saying. You're a student in high school. You want to buy a K20 and swap a Cummins into it. This would be your first truck and your first engine swap. You don't have a lot of fabrication experience, and you have a tight budget. I have a suggestion for you, but you're probably not going to like it. But I'm gonna give it to you in all sincerity, because I understand what it's like to be young and have a dream. So I tell you this with the advantage of years of experience. You aren't ready to take on this kind of a project. This is a huge undertaking and it's going to take significant amounts of time, money, skill or knowhow and resources to complete. I'm not suggesting you can't do it, anything is possible, but you'll need everything I've already mentioned. First and foremost, you need money. The purchase of a K20 may not be too bad, but you'll actually have to hunt down two different vehicles, and perhaps more for particular parts. The K20 should be in decent shape. Since these are older trucks, expect to have to make repairs to those parts you will be keeping, such as axles, and body work (No sense going thru all the work for a rust bucket) Next you'll need a Cummins donor. This isn't simply going on Craigs list and picking out a rolled truck. Cummins equipped Dodge trucks are rapidly increasing in value because these engines are becoming very popular, and btw forget the 4BT unless you happen to have a trust in your name, that you can cash in on. Next, swaps aren't as simple as dropping in a motor and turning the key and going. There are tons of details to deal with, everything from fitment to what are you going to use as a transmission, to making everything work together in one cohesive package. Besides money, that will take time, and a huge learning curve if you don't know what you're doing. And probably a ton of help. Over a period of time and building a bit of experience, this maybe possible for you, but you have to start were we all do. I would say, get a job in mechanics, better yet, in some sort of automotive/fabrication type of shop or join the military and try to get into something mechanically related, like aircraft mechanics. This will get you experience, also a free education and pay you at the same time. It will also give you time to slowly build resources. When you're ready, you'll have all the confidence you'll need to take on something this complicated.
There is another less difficult option, but you'll have to surrender your loyalty to either the Chevy or the Cummins. There are tons of diesel powered pick ups that have no need for a swap. They are more or less ready to go now, if you can afford the purchase price. You can either get a K20 with the older 6.2/6.5 diesel, or you can get a Dodge W350 or Ram 2500/3500 with a Cummins. This is a cheaper option, than a swap. You can then concentrate on improving the truck when you're ready, whereas with a swap, you'd be still building it, waiting for more money, so you can buy one of the thousands of parts you'd need.
Good luck
Ed