5.3 or 8.1 or even another 350?

[dman1982]

so I've been thinking when I'm done with this deployment that my truck may be due for a new engine, it has about 235000 on the original small block 350. I've seen a bunch of threads on here with the 5.3 and/or 8.1, i drive my truck everyday, not far but still a tank of gas only last a few days, but if i was to get a ebay special engine like the 5.3 or 8.1 how much work is it to put into my 84 k10? will my transmission bolt up to either of these engines(th350)? i really want to go fuel injection and this seems to be the route to go, the fuel injection kits are crazy expensive and i can get a whole new motor for the price of them. just looking for opinions/advice on which way to go