Given the length of the securing bolt, I wouldn't be too worried about that: a gentle tidy with a dremel or a file to ensure that it doesn't impede the caliper and off you go. Even better if you run Maguras as there is no pivoting action.
Because when i put the calliper on there is nothing to hold it in place 3mil at most is left so I'm going to try and put a disc brake on the back using this adapter £2.99 of Amazon if it works il buy the disc and back wheel if its crap I've only lost 3 quid.
I've not tried one of these, haven't needed to, but the reviews are good and it's only £7.99. I bought one of those disc adaptors that you're looking at, very cheap and nasty, haven't even tried to fit it.
A good fix and one that'd be (relatively) easily done and wouldn't involve any paintwork damage would be to cut it down flush with the bit where the spring mounts and then drill/tap the stump and fit a thread in post which are readily available in a couple different threads.