Welcome to the site, there are few members here that own Landy's (welding is all part of owning a Land Rover product especially a Disco Rotbox), which model do you have the pleasure of trying to keep on the road?
A rear crossmember isn't a hard job to do on a 90, but considering you've stated you wish to "improve" your welding, bearing in mind the possible loadings that could be applied to Landie's rear crossmember.
I'm not tring to be an **** but, how good is your welding at the moment???