At stage of panelling transmission tunnel and wondering what the pros and cons of either welding sides with mild steel sheet or rivetting ally sheet to sides? Using rivnuts and stainless/ally sheet for top.

Suggest you use ally panels rivited with 4mm rivits. If you go the mild steel route, you are adding to the weight and you will have the hassle of rust proofing them (paint).For the top panel, again you could use ally, folded over the sides and use either self tappers or rivnuts for securing through the folded over sides.
Stainless is nice but costly, obviously heavier than ally. If you scratch ally then you stand a chance of polishing out the scratches. With stainless it is almost impossible to remove scratches without putting in some serious graft. You would not be able to polish- blend into the surrounding area so the whole panel would have to be polished........reasonable with ally, horrendous with stainless. On a positive note stainless will take alot more abuse before it scratches and of course will not dull off like ally.

Hope this helps

Dave.

I welded 16g to the sides and painted it. It gets kicked and it's easy to paint it again. It's only a small percentage of the total weight. I self tappered the alu top on but have covered it with black vinyl now as it was dazzling me in strong sun, very distracting.

tahnks guys. Not too bothered about weight, have gone with mild steel and will do top in stainless covered with vinyl.

Good idea to weld it with steel plate: having welded panels on your transmission tunnel makes a significant contribution to the torsional rigidity of the car (less flex = better handling).

not sure how much this applies if it's just the sides and the floor, but it's certainly true if you do it all the way round.