measure your lifter plunger travel. you want shorter pushrods for .030"-.060" preload. maybe there is a factory spec for the 1970 340 six pack cuz it had hydraulic lifters and adjustable rockers
An average for most stock hydraulics is tighten till there is no clearance, then another 1/2 turn.
The ones in my Magnum roller are right about .040 and if it was worth going back in for it I would probably rather have 50 or maybe even 60.