I am restoring a vintage bike with a modern twist, i bought 3 new shimano half link chains a few years ago but they are too short individually so can i extend it by connecting the 2nd chain and using the quick links it has to use both ends once i remove enough links to work out its length, will this in anyway be dangerous or shall i not bother and just buy a super long chain? The Shimano looks great and very hard to get now in the UK so a shame to waste and would look cool on my 58t chainring on my vintage Moulton.
It's fine, i got several stretch cruisers running super long halflink chains (made of three BMX chains). The only concern you should have is about your chain link extraction tool. Since halflinks often are beefier then normal chains, it often happens they have thicker walls, so they might just not fit into a normal link extractor. But if you have some more serious tool, or if you are lucky and your cheaper one is wide enough, it would work. I have managed with a simple cheap extractor with one of my chains, though it barely fit, but then building another bike with halflink i just had to buy a new tool, and i broke my old one with the thicker chain

By the way, i can strongly recommend Super B TB-CC65. Such an awesome tool! Just a bit more expensive then the normal ones, this is so well-built and ergonomic! It is such a pleasure to work with it, and it provides lots of comfort and control over simple operations of putting chain together or apart.