I had small gaps on mine in a few places, it's no big deal if you are a fairly decent welder. I am quite new to welding and found that I blew a few holes where there were largish gaps. I soon learnt to fettle the tubes so they fitted as tight as possible and then to look at the joint before I started to weld it. By the end I got quite good and could modify my technique to still get a good weld if the joint was less than perfect. My advice would be to just take your time, make the joints as good as you can, practice welding first if you need to (don't know what your experience is) and most importantly, enjoy your build.
Welcome to the forum, there are some seriously hlepful and experienced builders on here, I would be nowhere near as far as I am with my build without their help.
Best and all that,
Enoch
www.deepfolly.co.uk