Can someone run me trhough the procedure for a 1964 273 mechanical adjustable valvetrain lash adjustment?
Do i set clearance at valve closed? or full open? hot, or cold? clearance specs?

You need to find out specifically what cam you have--so is this a stock, Mopar, factory '64 cam?

Most specs are HOT, but a few here and there are cold, but will say so

Set the valves "fully closed."

Most "cold" specs, --if posted-- are not as accurate as hot, because the different components don't expand exactly the same, I.E. the clearance can change differently between individual valve/rocker/ pushrods as the engine heats up.

Use the feeler as spec'ed and use a 'thou or two larger as a "go - no go"

Depending on how big the cam is, you can "fudge" the clearance between exhaust/ intake to slightly change the cam timing, but GENERALLY you do NOT want to use LESS clearance than the spec.

I have done /6's and 273's hot and running with good results. Slow idle speed down and use .013I and .023EX.

Thanks guys, yeah its a stock cam.

Or use Mopar's 8 position method as I outlined in this thread.

https://board.moparts.org/ubbthreads/showflat.php?Cat=0&Number=4571730

You need a lash setting.....the other fellow needs preload, but the order in which to set the valves is the same.

Warm the eng up thoroughly. remove valve covers & plugs. put a 1&1/4" socket,breaker bar & correct length of extension on front crank bolt. You adj each cyl when it is at TDC compression & a comvenient way to find that point is when it's COMPANION cyl is on "rockover". Split the firing order in half & put the 1st (4) #'s over the last (4) #'s. 1-8-4-3-6-5-7-2 so that the (1) is over the (6) and the (8) is over the (5) & so on. Rockover is the point when both valves of the companion cyl are MOVING & are at the same height on any given cyl,right then you adj it's opposite cyl. IE when #6 is on rockover do #1 & when #1 is on rockover do #6 then on to the next pair. It's more simpler & faster to do once you get rolling than it sounds. The specs are .013 intakes & .021" exhausts.closest way without using the specific valve gapping tool with a dial indicator which most of don't need is when for example(intakes) .012" feeler gauge is loose & .013" has a SLIGHT drag.

Before I went to the track last year Draginmopars set mine on my stock /6 to the hot specs, valves cold not running.