18 spline clutch kit choices

It depends how much power you are running through it. I assume since you got an 18 spline that you are making decent power. I ran a McLeod RST twin disc for one season and it was junk after one season. I then put a McLeod RXT twin disc in it and after one season it's perfect yet. I highly recommend the RXT. It has ceramic discs instead of organic.
It sounds like our set ups are fairly close. I run mine in a 74 Dart with a 4.30 S-60 with 760hp. It's 70% street and 30% strip.