If the lifters on the '56 are also hydros, then those would fit in your '58, as 56-62 are the same.
It's not recommended to use used lifters on a different cam, but since you're in England, and parts aren't readily available, I wouldn't worry too much about it. But new is definitely better...
Its not recommended to mix up solid and hydro stuff together, as the ramp on the cam profiles are not the same, but in a pinch, I've heard than solid lifters can be used on a hydro camshaft, but I don't really recommend it...
If you are certain of where the noise is coming from, you could just swap out those lifters, just to see if the problem persists or not.
There's no point to using oil that is heavier than necessary to do the job. Considering the climate in England, and that its a street car, a good 10W30 is plenty. Synthetic is up to you, but if the engine is a typical 235 and it leaks alot, it could be rather expensive to keep topping off with synthetic. Just something to consider...
I've always heard that you want to break in a fresh engine with regular oil, not synthetic. What kind of rings are you using?
