My old (MK1) .22 cal 77K shot best with JSB Express, definately edging it over JSB RS.
However, that was only from testing a single tin of each, so the results may well have been reversed with different batches of the same ammo. Such is the black art of pellet testing.
Personally though, unless grouping proves otherwise, I'd stick with lighter weight pellets in a springer. They're often more efficient in a factory standard gun, and generally give a nicer shot cycle.
Modern .22 Superdomes are near the same weight as JSB Express - but they're comparitively crap, with a crap BC. So I wouldn't bother too much with them. Unless they go particularly well in your '77 barrel.
Depends what you want to do. If you only want to do short - medium range plinking, to be fair, most pellets would likely be OK.
Have fun with your '77.