The part that bothers me is that he's taking what looks like a numbers-matching 6-cyl car and "restomodding" it with a modern engine.
I mean, forget the fact that you need to brace the unibody on a 6-cyl car so the torque from the V8 won't warp the frame; that's $300-$500 worth of parts and you can install the bracing in a weekend. What concerns me is that he's modifying an original 6-cyl car.
The same reason I kept my Fiero all-original--and was relieved to sell it to someone who wanted to do the same--is that so many people modify the hell out of them that all-original cars are going to become thin on the ground. I'd bet this guy could have found any number of already-modified cars which needed little to no bodywork (same as this one) which he could have done the same thing to, and been no less satisfied with the outcome.
And by the way, the price of that car is why I was so shocked that Buttercup happened at all. I expected that car's asking price to be a lot more than it was.
Anyway, that post there has some useful ideas for changing the seat mounts, which is why I wanted to link to it in the first place.