yep 1mm 18gauge is about right.
Probably find that because the sill is a closed box section that Frod would have used 20 gauge which is about .8mm
Tinsnips are fuck all use on this sort of stuff unless they have 2foot handles, scribe a line and use a 1mm cutting disc. Don't cut right on the line you can always use a flappy disc to trim it back a little.
My local body shop has a plasma cutter which they let me have a play with, "gotta get me some of this" I thought. cheapo 240v units don't cut the mustard though. Now I use 1mm cutting discs and a long handle shear like this https://www.frost.co...less-shear.html
shield everything within 5metres when cutting. those pretty sparks burn through paint and make nice rusty pin spots all over the paint. They also seriously fuck up glass, and upholstery etc etc, I used to have an old rug that was quite good at laying over the top of bits that needed protecting and not so thin that the spark would go straight through it.
When welding try start the weld on the newer thicker steel and draw the pool onto the old stuff. less chance of blowing holes through all over the place
p.s. you forgot celebratory beer for when you hit milestones, dont drink and weld etc etc wont someone please think of the children
Shitroen C4 GP 2007 1.6HDi - Wife's daily written off, farewell trusty but slightly unpredictable steed
Pug 307 - son's daily runner Sold to Arthur Foxhake
Range Rover Vogue 1993 - my daily runner
Daewoo Matiz 2002 - Sold
Jaguar x308 Sovereign 4 litre - nicked by Fathathestag might be up 4sayle soon
Triumph Staaag 1973 - cheaper than a mistress.. just
Range Rover 1972 - ongoing extensive restoration
Range Rover 1972 - complete shed but in line for restoration
Land Rover 1969 SIIa SWB - belongs to Fathathestag but maintained and welded by me
BMW E46 318i touring - sold to hhhugues