Well some wisdom to ponder on. (I have a few years on you.)
1) It's ok to make a mistake, as long as you learn from it and do not repeat it. So do not beat yourself up for making this mistake. Just be wiser and don't do it again.
2) Don't tell you kids you went romping, almost rolled it, and bent up your truck and killed it till after they are teenagers. They will use it as their excuse to do the same.
3) Time heals. I guarentee you won't feel so bad as you did the day it happen, a month from now. Even less two years from now!
4) Your first motorcycle, your first truck, and your first house you will make mistakes doing repairs and have regrets. It is how we learn and become better. So take what you now know and make a bigger and better truck. I see your are sentimental about this truck, but its totaled. So take the escense of it to the next truck. The engine you built with gramps. Perhaps the seat you both sat on. The rims and tires or the tailgate. Maybe that cracked rear view mirror that reminds you of something that happened. And use that to build a new and better truck! You just got a do over, so sieze the day and do it!
Let's face it, the engine and tranny are the hard parts to rebuild. The rest is pretty basic in comparison. So having a engine to swap is a great start to a new improved truck! Know your grandpa just wants to see you happy, he does not care if you buy a new truck, he like all of us is just glad you did not get bent up like your truck! You can buy a new truck, you cannot buy a new back!
I look forward to the next chapter in your "6.9 Project Thread"!