1973 Datsun 210 which would be better

I'm assuming you don't know much about cars yet or you wouldn't be asking these questions. Why are you pursuing a project that few professionals would get involved with? GM made some really popular engines. The 305 is not one of them. No professional is a fan of sticking one V-8 into a different brand car body, especially one with a body and suspension system that was designed to support a four-cylinder engine of one third the weight. The engine will not fit physically with the transmission that will have to be used unless the body is chopped up which will compromise the structural integrity. The cooling system will have to be beefed up. People forget about the suspension system that has to support that added weight, AND allow you maintain control when cornering. How are you going to modify the brake system to restore that to work safely?

Lawyers and insurance investigators love to find things like altered ride height and cobbled electrical systems. They will convince a jury that you were partly at fault for the crash when the other guy ran the red light, because you were less able to avoid it, ... And they will be right. They will have a field day with this car, assuming you can find the parts in a salvage yard and get the car on the road.

You also have to remember there's a reason the current owner didn't finish the project. These cars usually end up with a "For Sale As Is" sign in the window, and the seller is relieved some sucker came along and hauled the nightmare away.

Jaguars are expensive cars built with expensive parts that are expensive to replace. When they don't need repairs they don't do anything any other car can't do, so the only reason to want to own one is because few other people will have the same thing. Unless you have some reasons or desires you haven't mentioned, I'd stick with a common car that doesn't need work that experienced mechanics wouldn't attempt. The Datsun is hardly a desirable older car, and few people have any interest in owning a 305. The only positive to the Jaguar is it's old enough to not have many computers. My daily driver is an '88 model. I trust it a lot more than my newer vehicles.