Enoch,with arc welding,keep all gaps to 25 thous or less,this is easy to achieve and you will become a very good fitter real fast if you keep to that standard,also bevel each join where possible to 50%.
Use 2.5 mm (14 swg) rods or smaller,usually general purpose say GP 6012 or similar.
If possible try to get some 2mm (if available),shop around,they used to be sold under the names of mirrorcraft/satincraft.
If its possible to rotate the chassis for final welding,(a rotating engine stand for each end will speed up the build ) then you can with general purpose rods use the vertical down technique and produce very high quality welds,you will surprise yourself how neat they will be.Use your left hand to steady the rod as you come down.
And this applies to ALL welding and gas cutting(especially gas cutting)
ont hold your breath welding,common mistake,learn to relax and breath normally.
Final finishing,remove ALL slag,tidy up with a disc grinder and round the sharp weld edges with a 1/4 or 5/16 chainsaw file these larger saw files are not generally stocked by chainsaw dealers so a 3/16 will do OK.
AND AGAIN! 25 thous,the approx thickness of a hacksaw blade is the max gap to have on all types of fit ups on these thin sections,it will soon become 2nd nature to achieve and will stop you from trying with MIG or Arc to fill gaps due to sloppy workmanship.
Arc welding is cheap,fast and very strong on these tubes once you learn how.
Give it a go!David