Any advice for novice welder about to stitch new rear cross member to 90 chassis?

Good evening everyone.

My 92 200tdi Defender 90 hardtop has just failed its MOT for a rusty rear cross member. It's not all that bad and its encouraging that the rest of the chassis is actually pretty solid but I figured I'd make life easy on myself and just fit a new rear cross member. So I've ordered a new galvanized 2mm item with the sleeves to slide over the existing chassis legs. So I'll be replacing everything from a few inches behind the rear spring turrets.

This will be a challenge for my welding skills though. I've fabricated and welded seat frames up which seem to have stood the test of time, and ive done a few repairs on body panels to other vehicles i have but this is a major structural component that I will be hanging a Ifor trailer off fairly regularly.

Of course I'll be having a few trial runs using the good metal in the old crossmember before I begin to weld the new cross member into place, but what tips have you got or what threads can you direct me to which will help me to get my welds done right? I'll be using a Clarke EN151 Turbo with Flux core wire, but what diameter wire would you recommend? Should I weld in a "zigzag" pattern along the seam or just keep the weld pool coming along in a straight line? How long a weld could I make in one go before i risk getting the chassis legs too hot?.....
These are the kinds of questions I have on my mind.

Any advice appreciated. I may not have a huge amount of welding experience, but I am confident I can do this job well with a bit of thought.

Welding is not one of those things like riding a bike, [ aircraft welders who have more than a couple of weeks off have to do a practice/refresh before restarting on the job proper.] So doing a good bit of practice on metal same thickness before starting will be good.
When seam welding I like to use a zig zag style of movement as working, actually more of a quiver of the hand/torch as moving along the seam, Several short runs around a joint [ inch] before joining them all up. Important that joints connect with no gaps and galv has been cleaned off at least half an inch back from weld area and any paint, rust on the chassis stumps.
Not a fan of flux core welding, not good doing thin stuff [ I do both Stick and Mig welding] you do not get the cooling like with the gas.
Have a look at Mig Welding Forum. Lots of good information for beginners.

That's fantastic, thanks.

Maybe I'll try mig. The reason I suggested FCAW is because I'll be doing it outside and I do run the risk of blowing my gas shield away with MIG. Maybe on a still day with a windbreak I'll get away with MIG.

May I ask what mig wire you'd be running for 2mm steel?

Also, I may be overthinking this now but what are your thoughts on cutting "windows" in the under side of the new chassis leg sleeves and welding a seam inside the rail where the new leg meets the old leg, and then welding the window up afterwards? This would be done as well as welding the outer face of the chassis legs too.

Would it be worth going to those lengths or not?

I use 0.8 wire in my Murex welder. It has 5 power settings and a vairy speed wire feed. This feature helps finding the right amount of wire for the thickness I am doing. On stuff above 3mm I usually revert to my old stick welder I have used/owned for 54 years. First thing I bought when I left school.
Have done Mig out side [ some things needing repair will not fit in workshop] Like you say a little wind day, a wind brake, even use bits of card to block gaps wind comes through, turn up the gas feed on welder.
I would not worry about "windows" just make sure you have proper penetration [practice on same thickness steel off the vehicle both flat and vertical] Some good videos of defender crossmember on the you tube.