If you are trenching high voltage lines, you may have a local code that tells you how deep you have to go. It may be something like 12"? A middle buster for a small tractor is only going to go about 6" deep. A sub-soiler is going to go roughly 10" to 12" deep depending on the brand. I think it might be less work to rent a trencher than to use a middle buster. The trencher will leave you a clean trench. The middle buster will leave spoils in the trench that you will have to dig out to lay down your wire. As you are doing high voltage, you will probably want to enclose it in conduit, which again would favor the trencher for the task. If you could get a sub-soiler instead of a middle buster, then you could spend a hour or so rigging up a cable & pipe puller as I described above and get your wire down about a foot.
Personally, I'd probably rent a trencher unless the land was flat and open and I wanted to build something out of my sub-soiler. But if the choice was a middle buster or a trencher, I would choose the trencher every time. Realise too that you are working a fairly short run of wire, and you have to hand dig the beginning and the end (the length of the tractor to the wall) so you might find it much easier to do with the trencher.
The thing I built, and the thing others have built is great for long runs, but it is not great in smaller areas and is a pain when you get up to the ends because of the hand digging.
As for the 755, I'm sorry but I'm not familiar with that model. If it is a 2 wd tractor roughly similar to a 790, then I'd say you could pull a sub-soiler/middlebuster through most soils.