Dual idlers can not compensate for any deviation in crank height from line honing. If the block was line honed then the fit will be tight - maybe too tight. Or left stock it could be too loose. Years ago I had a Milodon style but it could have been a Donovan or KB. It had an adjustable idler. The problem was the idler was a different diameter than the other two gears so if you started dialing in the cam with the dots lined up, they didn't line up again after one crank revolution. If you forgot how many turns you'd done you had to remove the sprockets to line the dots up again. I gave up and switched to a chain. With the Milodon style, water pump clearance will be an issue and then the crank pulley will need a spacer.