I see a Dana....
My trunk floor in the 70 Charger looked similar to yours. I did just what you are doing...cut what is bad and replace only that. I also lap welded my donor floor panel.
I blended mine fairly well with careful grinding of the welds and some body filler. Regarding your welder, I have to mention that any flux core type MIG welder is inferior to a setup with gas/argon/C02.