HZ bumper bar gap

On the left side you can see that the bumper return end is sloping down. That would indicate that either the bracket or the surface it's mounted on is bent. Needs to flick up a little.

I used to take the bar off and use a length of timber against the bracket and jack it up from there.
Elongating the holes could work but would always run the risk of it slipping back to the old position

Find a better panel shop. They could have done a perfect job if they tried hard enough. You pay peanuts you get monkeys.

You don't want to elongate any holes. The holes in the brackets to body are threaded, the holes through the body are long and if you elongate, they will leak water into the boot and finally the holes holding the bumper to the brackets are square holes. As oneton said, find another panel shop. There is enough movement in the bracket attaching bolts to get that gap better. Another reason for the gap if unable to fix is previous bad repair.
See attached picture of your rear bar for what I'm talking about.

Feel free to post more pics mate 😁
We all love a good resto

Take the bar off and where it bolts from the bracket to the bar it's self you can muck around abit be careful of the bolts as they snap easy but .The slots in the bar are for the jack and if the car moves while changing a tyre can cause this drama.
The bar it,self is weak and will flex you need to get the brackets straight! Or put a washer or two in between where it been pushed.

Under the bracket on the bent side should angle it up enough.