There is one Simple way to do. Restore the backup you took from BlogSpot and then Migrate in WordPress and set Your Default permalink structure like you want.
This will change the permalink structure in all posts which aren’t a good part to be considered.
Now here come’s the main part, You will have to edit your each post and edit the slug of it. You have to make the slug same as it is made in Blogspot, see the below screenshot to refer as an example.
Edit this way in your old post as you see below, Just an example. You have to copy the same URL which you already have in the search engine so that people get your post without an issue.
I know this is some hard work to do. But, this is the best way to avoid losing Ranking and extra useless URL for the website. I hope this is a useful post for you all.