I have welded lots of cars in the past ( and Landrovers) I am a fab/coded welded so that helps. Its not rocket science but you will need to take your time to get a good job. As someone else has said, make lots of small cuts and keep doing this till it is a good fit.
On the welding side, make sure you get it all as clean as you can. It sometimes helps to tack small lugs to the box section when joining because it will help line them up.
When your ready to fully weld after tacking it all in place, try and weld from the bottom up and not top down. This will give a much stronger weld.
If your not sure or need any other info/advice on this PM me and I dont mind giving you my number to talk it through.