Welding will distort the steel. It'll no longer be straight. Steel is pretty cheap. Find a metal supplier near you (not a big box store) and buy the angle you need.
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Agreed. You could even get tighter tolerance pieces off McMaster-carr
Calling me a metalworker would be an insult to real metalworkers everywhere. That said, depending on the size of the piece of steel, it’s probably not worth your time to fill in then grind flat when local big box hardware stores sell common sizes of “welding steel” for relatively cheap and as the other poster wrote, McMaster-Carr has just about any size piece of angle you can think of.
Source: was a kid with no money scrounging in what was left of grandpa’s (tool maker) machine shop who is now an adult and now buys the parts I need rather than trying to make do
Much easier to find a new piece of steel to use.
Like someone said above- the amount of heat required to fill those holes will most definitely warp your steel.
You could grind down any burr on the holes and let it be with the holes. Not sure if that would still suffice for your requirements though.