Been debating about either buy a welder and weld it or is the 3m structural adhesive and maybe a few rivets good enough thanks? Will be the front drivers side if that matters.

Weld it, it's a unibody, it will be much stronger if you weld it in. You dont need to have a continuous weld, "stitch" weld it every inch or so, seam seal it and your done.

don't think you're allowed to pop rivit on a vehicle anymore, it may fail inspection.
3m adhesive is allowed, so is welding. not sure if screws are allowed anymore either.
the adhesive would work for you if you can't have them welded in, then cover with bedliner or similar.

I am going to be doing this soon. But my question is, when welding do you do a lap joint or a butt joint? I have yet to even hold a welder in my hands, but I have access to one. This will be my first go at it.

On floor repair, I always do a lap joint. I'm pretty sure the replacement floor pans are designed to be installed that way anyways. It's been at least 18 years since I've had to replace a floor pan ( now that I don't live in a New England....lol)

I will be doing this soon for a friend though , he has a 95 that's needs both front floor pans. Should be fun....lol

Lap weld is easier. Butt weld is better. Lap weld creates a void space (where the two panels lap) that you can not paint, you can only seal it. Water will eventually get in that space and rust the joint out. Butt weld if you plan to keep it for 20 years. Lap weld everything else. Butt weld is not a beginners project, very difficult to get a good fit up and weld joint. Lap weld is super simple. In your circumstances, I would recommend a lap joint.

Well went ahead and ordered a welder (Esab Rebel EM 215ic) plan on slowly ordering supplies on a weekly basis ...cutoff wheels,all types of wire brushes,bottle of gas and others stuff since looks like I will be working overtime for the next 3-4 weeks. Plan on doing the plug weld (self tapping screws to pull tight then removed and weld the holes up)

Flux core wire is good to use, then u won't need shielding gas. Get some re rod cuz its cheap and grind a point on one end and make it about a ft long or how ever long suits you. On the other end weld a 3 inch pc across the non pointy end. U can place the point on the top sheet of metal where the two sheets of metal need to be tacked and lean againt it with your sholder while u tack it togeather.

Thought about flux core wire but with the thin gauge metal figured best to use gas and some .023 and avoid burn thru, still might try .030 flux and see what happens.

Flux core burns hotter. Stick with the gas and use as thin a wire as possible.

BTW, post up when you use that ESAB welder, never used one and curious. I have the ESAB Sentinel hood and LOVE it

I sure will but it will be about 3 weeks or so.

75/25 gas and .023 wire is your best bet. I welded new floor pans in my Camaro last year. Butt welded them. Used .023, welded well.

-2000 Cherokee Sport 4WD
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Mine has the same problem except its on the front passenger side.
There was a giant hole where you would typically rest your feet as a passenger. Luckily all I had to do was order a new floor plan since the frame wasn't rusted out. I have someone welding it in for me sometime this week.
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Greetings from Ohio.