To reduce warping of the metal from welding, which is worse in armour plate I am told, try button welding.
Start by prepping the metal as you would for a normal weld. Then when the sheet is in proper alignment tack weld both ends. Now get out your tape measure ( the thingy with the numbers on it
) and mark every two inches front start to end. Okay, so now you have a line of marks starting at 1 then 2 then 3..etc .
Now tack weld all of the marks.
Now go to tack number two and weld back to number one, then go to tack number three and weld back to number two and then 4 to 3 and so on. Make sure you stop and peen each weld before proceeding and if the temperature starts to build up in the metal , go for coffee. You will get an incredibly straight twp pieces of metal with no need to start with straightening techniques, though it does take considerably longer.
Sean
Oh, and high nickel rods are common but more expensive on this side of the pond. So are stainless rods and they would likely do a nice job too.