Reading through a few old threads, is the concensus that a decent spec for the alloy panels would be 1.2mm on the sides and 1.6mm on the floor (all H22 5251)?

Al

Looks correct to me, I boxed the side forward part of my transmission tunnel with 1.2mm steel welded in place, makes access more awkward but adds strength and far better protection should the prop fail.

Regards Mark

I've personally gone for 2mm for the floor, side panels same as yours 1.2mm ali.

Sounds about right. This is what I went for on my Haynes:

2.0 mm - 5251 H22 for floor and seatbacks
1.0 mm - 1050A H14 for side panels, rear tub, and tunnel panels.

I've heard of people struggling to make side panels out of 5xxx ali. 1xxx series is a lot softer and easier to work with if you need to bend it at all.

i used 1.2mm Ali for all the panels
i used 1.2mm steel for the floor
and i welded 2mm steel plate on either side of the prop tunnel (only the center Rectangular pieces)
i have seen a prop break on a dyno and put itself though the ali panels nearly taking off a mates leg. he got off lightly with a few cuts and bruises

If you use NS4 1.2mm sheet is good for the floor.