Do I need to install high flow catalytic converters if I put long tube headers on my truck?

This is not exactly correct. Long tube headers will change the back pressure in the exhaust and create lower end torque.

So, if you are going to go that route then I would suggest adding high flow cats not because I would worry about sucking things back up but because you will want to ensure you limit the amount of back pressure that can hang in the headers.

If the back pressure builds in the exhaust, you will actually decrease your HP because you are not able to get the air out quickly enough.

https://www.2carpros.com/articles/catalytic-converter-replacement

So, if it were me, I would go with a complete exhaust system if you were going to go with long tube headers and this will help you maintain as much HP as possible and benefit from the torque increase.