For the correct valve timing for ford EA falcon 6cylinder,4L is piston 6 or 1 rocking and does piston 1 or 6 need to be top dead centre

What exactly do you mean by an "overhaul"? More information please. Which diesel engine is this? The 2.5 litre or the 2.6 litre? What year is your vehicle? Some engines can be different settings depending on the size it is.
I'm assuming this is for the clearance between the valve rockers and the spring end of the valves?
If you're breaking rockers your mechanical timing might not be set correctly risking bent valves or the valves are already bent. Even One tooth out on the timing belt/chain can cause problems. You might need to take the cylinder head off to check the valves.
When you set the clearances the engine needs to be Cold and the cylinder you're working on needs to be at Top Dead Centre (TDC) to make sure the valves are closed. On an inline 4 cylinder engine the cylinders work as pairs, 1 and 4, 2 and 3.
Set cylinder number 1 to TDC. Set the clearances for only cylinders 1 and 4. Using the crank pulley bolt turn cylinder 1 to Bottom Dead Centre, (180' turn clockwise). This should bring cylinders 2 and 3 to TDC so you can set those clearances.
Is the engine burning the oil? (a thicker, more blue exhaust smoke than normal). Burning oil can also be a sign of a blown head gasket or badly worn piston rings.

there is a method but you will have to have an understanding of valve timing
in an engine regardless of number of pistons when number 1 piston is TDC not firing, then the exhaust valve will be open and ready to close and the inlet valve will be closed and ready to open
so bring the piston up and near tdc
as you approach tdc you will see the exhaust start to close and at the same time the inlet valve will start to open
if you rock the crank back and forward slightly at tdc you will see that the valves are rocking
this is valve rock and indicates the cam is in time with the crank
it is only necessary to do it with no 1 piston
It doesn't matter how many cam shafts ( one cam shaft can be inlet valves only and the other can be exhaust valves only) there are, it is the exhaust and inlet valves for no 1 piston that you are looking at
how do you get tdc-- remove a plug and with a piece of wire place the end on the piston crown and as the piston comes up the wire will be pushed out as you rock the piston the wire will move in and out and the point before movement is tdc
probably as clear as mud unless you are engine savvy

all engine timing for valves and ignition is set at top dead centre of the pistons ( piston fully up in the cylinder)

the piston has to be on tdc
the valves have to be in the valve rock position
that is the exhaust is just starting to close at the same time as the inlet is just starting to open
the number of links should now be at the timing marks or the timing marks should line up when the crank is turned one full revolution

It doesn't matter where the other pistons are as long as #1 is all the way up or Top Dead Center. What matters is what the valves are doing when #1 is at TDC. Watch the valves carefully as you turn the engine over and make sure that the intake opens first. At TDC both valves should be closed and as you go past TDC the exhaust valve will start to open. Remember that this only happens in this order every other time #1 hits TDC.
When you have this order correct the #1 piston should fire. Hope this helps.

if its a straight six or four cylinder if no six valve rocking then one is on top dead centre or four cylinder 4 rocking one is on t dead centre

just because the piston comes to the top does not mean its on compression stroke. the piston comes to the top on the exhaust stroke aswell !! if ur out by a single tooth it wont start. take of ther timing belt cover and the 'rocker cover'. ensure no 1 at top dead centre making sure valves 1 and 2 are 'rocking'. both valves must be shut(as in up not down) for a compression stroke !! refit ur distributor ensuring its perfectly on no 1. rebuild and bobs ur uncle. ps u may have choked out ur plugs starting up like u did, make sure they are clean. good luck.

Both valves should be rocking But only Inlet just opening, and Exhaust just about closed and its the same on all cylinders..
When piston goes down it sucks fuel mix in and almost a bottom of stroke it closes..

Yes the harmonic pulley has to be in the correct position. usually the pointer should be at the correct timing mark with the #1 piston at top dead center. The cams have timing marks on them also that have to be in the correct position, this is extremely critical because if the cams and the crankshaft are not in the proper position the pistons can bend the valves or the valves could crack the top of the piston if the engine is turned over with everything not in the proper timing sequence.

With the rocker cover removed , if you look at the back of the camshafts you will see each one has an off centre slot. The way to get them correctly lined up is so each slot runs parallel with the face of the cylinder head. There is a special tool to lock the 2 cams together which is simply nothing more than a flat piece of steel. If you can not borrow this 'tool' the other way is to make sure each cam lobe on no4 cylinder is facing down  and pointing in on the valve tappets.