The Pete Jackson type dual idler gear drive is very forgiving. It does not need to be dialed in within mils. I ran one in my very first build '73 Monte (built in '76) with solid cammed 350 SBC. It didn't use a thrust bearing (relied on the one in the block) and no button was used. I blocked the back of the water pump to bolster the cover. I drove the bageezus outta that car. I was immature back then though . The gear drive was obnoxious to say the least.

I will never use another one

Kitz