You won't need any kind of converter box to make it work if you have tuning software, which is needed to remove VATS anyway. What you need to do is change the tach resolution from 6 to 3 in the tune to change it to 8 cylinder instead of 4cylinder. That will make it read right, but only if it works in the first place. To make it work you will probably need a pull up resistor to strengthen the tach signal to the tach as some PCM's don't put out a strong enough signal to work an older tach. LT1Swap has a good write up on how to do this.
https://lt1swap.com/lsx_tach.htm