I Agree with everything above...how i learned to weld was putting in rocker panels on a 68 chevy pickup..and replacing the doors..the mans welder i was using took my old doors and began to drill 3/8 holes in the door..about 50 of them..then he told me to weld up the holes....he let me ( on purpose) start on the 1st hole and i made the hole 4 to 5 times larger than what it should have been..anyone ever weld a light gauge metal with wrong heat range and wire speed can relate to this...then he showed me the proper way..heat,wire speed and also how to stay on top of my weld and work it...i got pretty darn good at welding up holes,patch panels and some roll cage work...
i will give some advise on here as well..i agree with the PRACTICE,PRACTICE..even the most experienced welders out here get a little rusty on their welding if they have had long spells in between pulling the trigger every day...so YES PRACTICE..
Next take the extra time and effort to properly Notch and fit your tubing..the tighter the tolerance on your notching means its much more forgiving on your welding...if you have a huge gap in between tube notches start over and re-notch...and chip away a little material at a time..you can always take more away but you cannot add back..learned this on floor pan installation years ago. 8)
the biggest mistake beginner ( novice welders ) find themselves is COLD WELDS....this is a weld that looks decent from the outter eye..but in reality it never SEATED into the metal..improper heat range and wire speed can cause this (very dangerous cause it looks like a good weld but no stronger then a tack weld in most cases).....also WITH A MIG welder PUSH the bead instead of pulling it...you can also Hear WHEN A WELD seats or Burns INTO THE METAL...UN-MISTAKEBLE SOUND...if its popping and carrying on stop..clean your surface, re-check your heat and wire speed..
Now Tig welding gets real fun......lol...
good luck...also i would be more concerned in getting the car laid out right more so than i would the welding at this point...so i say consult with the pro's on getting the car laid out like it should be ..unless you have a JIG and i still say consult with someone who knows their stuff..but looks like your gonna have to square the floor and build it from therewhich can be done and has been done many times...
a goofed up weld can be fixed little to no problems...you build a crooked car in every aspect..you will never get it to work as you would like...and possibly piss away a lot of time and money..
but that an individual call..
good luck any route you take my friend
and Merry Christmas
Brian