I am going to replace the tire with a 3.50
You might want to take a look at the recent "Cheap Tires" and "Replacing Tires" threads on the British Motorcycles In General Board. Domiracer sell a 3.50x18 but, like most small Imperial sizes, it's Far Eastern-made.
Otoh, 4.00x18 isn't really "oversized"; if the US-market '64 Bonny wasn't fitted with it originally, Triumph went to the size (with a 3.25x19 on the front) shortly afterwards (albeit fitted on a WM3 rim) on US-market twins and carried on fitting 'em 'til the mid-1970's. Also, those are the two Imperial sizes available in modern tyre ranges from premier makers like Avon and Continetal.
Otoh2, you could consider a 100/90x18, which is about as wide overall as a 3.50x18 but has an overall diameter about 1/2" smaller.
and replace the security bolts.
In theory, they should've been easier
to fit in the bigger tyre?
Only reason I can think they were left out was the p.o. couldn't deal with 'em. If you do a forum search with keywords like "Rim Lock", you should come up with plenty of advice on how to make sure they aren't a pain during tyre fitting.