Hmmmm..........
Ok, I will start off by saying welcome to FABO, and I hope you are doing something that you are going to enjoy.
You have stated that you are in high school, work part time, and the car is a weekend project that gives you a chance to spend some good quality time with your dad. All great stuff. I am trying to remember being 17 and in high school, working part time, those were some fun times. I had a lot of great plans then too, but reality for me was most were far from my reach financially. It sounds like that is less of an issue for you, and that's great.
I read your thread on dropping a 440 into your 67 Barracuda and was starting to get the wrong idea about what you were looking for.
Reading this thread, I think you have accepted the reality that the amount of money and work you would need to do to get the 440 in and running is a little more complicated than it seemed, so a small block is the better way to go. As for the engine, a 340 should be an easy near drop in replacement for the 273, so that's a big problem solved. A package deal including a tremec could be a great deal, but as many others have already stated, the transmission will require a lot of fabrication to make it work in the car you have. Why not sell the tremec, or keep it for yourself for a future project and go with an automatic transmission? A 904 can be built to support a 340, and should be what came with your 273, so you know it fits. You could also use a 727 with a little bit of "clearancing" for fitment, but if you still have the 7 1/4 differential, it won't take long for that to grenade. Upgrades there can be an 8 3/4 Chrysler, an 8 1/4 Chrysler, or an 8.8 Ford sourced from an Explorer, search engine will help you find threads related to this.
Bottom line is that it's your project and your money, don't let others tell you that you can't do something, but you will be best off by keeping it simple. You sound like you have a lot of sense, but need more experience and the only way to get that is by doing.
I took auto shop in high school, like many others here on FABO, and I have learned from experience. One of my classmates built a 1974 Chevy Vega, powered by a stout 350 V8 and automatic transmission, in class. Including building his own framerails with 2x3 inch steel tube. I helped him pull the original 4 cylinder engine by hand in his parent's driveway, they didn't even have a garage. Sure, he had help from older friends who had done it before, but he did most of the work himself.
Good luck and keep us updated on your progress George.