New[ish] Red Tailed Boa

[WrongPython]

First of all, good on you for your thoughtful efforts and trying to do your best by this boa. It's quite refreshing to see people go to such lengths for reptiles.

Secondly (and this is the something I agree with bns on): at this point, Thor is Thor. His personality is what it is and you can't "change" him. If you'd like to take him in, you'll have to learn how to go with him and adjust your keeping to fit his needs and personality. If he truly is a defensive snake that prefers to be left alone, his wishes should be respected. Frequent handling to "calm him down" will only cause him undue stress.

That being said, are you sure that his "defensiveness" is actually defensiveness/aggression, and not just him being food happy? Some boas have an overzealous feeding response and will reflexively strike whenever their enclosure door is opened in anticipation of being fed. For them, the door opening is pretty much the dinner bell ringing! This is why tap training is so important with snakes - in short, it teaches them that you coming into their enclosure does not always equal food. Based on what you've said so far, it sounds like Thor isn't so much defensive as much as an overzealous feeder. Overzealous feeding responses are something an experienced boa breeder would know how to handle, hence why Thor became "defensive" again after the breeder passed - his breeder might not have thought much of his behavior, while people not as experienced with such boas do.

Third: are you sure Thor's actually eight feet? Even for a power-fed male common boa, that seems a bit large. It's pretty easy to overestimate a large boa's size when you're "eyeballing it" or measuring from an old shed. Try having him stretch out straight against something of known length (ie. a section of wall) and seeing how he compares. If he's more in the 6-7' range versus the 8'+ range, then a 6' cage should be alright for him. It'll give him enough room to stretch out, get comfy, and get some exercise in while being more manageable than something "huge."

In general, the combined length and width of a snake's cage should be no shorter than the snake's overall length. In an ideal scenario: length of cage (length of snake) X width of cage (half of length of snake) X height of cage (half of length of snake). Personally, I'd argue that a snake who spends the majority of their time in their enclosure (ie. one not taken out for handling for one reason or another) should have a cage of the "ideal" scenario - since they won't be coming out for handling-based exercise, they should be given more opportunity and space to exercise in their enclosure.

I hope this helps! Please take my advice with a grain of salt, though. I'm just a novice keeper - a well-read novice, hopefully, but still just a novice - so you'll want to get advice from more experienced keepers as well.