Just get a BBK and stop trying to reinvent the wheel.
If you can build the wheel cheaper why not do it? I will always build before I can buy when feasible.
I did some more digging and it isn't feasible in this case though, 4.6 mounting pattern isn't square like the 5.0.
I get where you are coming from - personally, I love building stuff. If given the option between building something or buying it, I'll gravitate towards building it every single time. With that said, and where ReplicaR is coming from, is that sometimes it's just not worth it. Everyone wants to save money, especially if you are the type that likes to do things yourself, but I highly suggest getting into a habit of trying to determine if it's actually worth it. I've done many things to cars over the years, and when I started I was like you (not trying to imply that you are "just starting", I'm just saying I was like that). I wanted to do everything myself, save as much money as possible. As I grew older (and I hope wiser), I realized that some stuff just wasn't worth it. Rather then spending time on some of the things I did, I should have just bought the part and moved on - I would have gotten more done on the car and I would I have enjoyed it more.
Now that I have the Mustang, I try very hard to employ this philosophy: I do as much as I can myself so I can get that satisfaction, but I also take the easy road and spend the money sometimes because I want this thing done so I can enjoy it. The hard part is determining where that line is. I applaud your desire to make something work - but in this case, just buy the 5.0 part and be done with it. It's not worth your time, or the small amount of money you might save. Put it in perspective: it's just a throttle body. No big deal...now, if you were talking about a turbo kit for example, then yes, please do obsess over the details there, because that will time well spent. But for a throttle body? Naw. Find a good used one if you want to save cash, call it a day. The challenge isn't just building something, the challenge is also deciding when it's the right time to build something...and when it's not.
Just my unsolicited advice from experience; if you choose not to take it, it will not hurt my feelings