Is a oil slinger on a big block necessary?

no but if you have one I'd use it. there was a debate awhile back whether it is an oil slinger or there to help protect the seal & seal material is much better than decades ago. If you need one I'll get you one in your mailbox for a ten dollar bill!

After I installed the motor back in the car and reinstalled everything I realised I had left out the slinger. Just dont want to undo it all. Its just going to be for cruisin and touring. Thanks

You'll be fine

Don't worry about it. The factory quit installing them without changing anything else. It will be just fine.

I have always had good intentions to put them in and somehow have always left them on the bench, never had any problems. When I built the stroked 4.0 for my Jeep I did the same thing. I cleaned the oil slinger and put it in line to be installed, ended up on the bench just like my small block ones always have LOL. Its got 50K miles on over 11 years, I think its going to be OK.