Neither my Block or my donor engine have a dipstick, in the massive box of spares i have 3 - 2 are marked 4.2 (but are different length with the L and H at different "heights" and the other is unmarked except the L and H markings...

Should I just pick one? or not risk it and try to find the correct one?

 

I would prefer the correct one for the engine. That said my 1967 S Type engine has a dipstick which shows 3.4, 3.8 and 4.2 but there is only one set of hatch markings for the oil level.
The tube that the dipstick sits in is large enough in diameter for the thickest part of the dip stick to pass through but the hole in the block is only wide enough for the lower section of the dipstick to pass through so the length you need to be concerned about is only th lower section with the flat on it. If this is th same length of all the dipsticks it does not matter which one you use as long as you can note how much oil is in the engine. The oil capacity is 12 imperial pints or 14.5 US pints or 6.75 litres. If you put this amount in the engine, let it settle then check the fluid level on one of the dipsticks this is the level you need to maintain. A small dab of paint on the dipstick to indicate the correct level would then give you a mark to always check it to.

 

The oil level is much the same for all the XK engines, but the dipstick tubes are different lengths and the dipstick has to match the tube. I think the tube may be shorter on some of the sports cars, but most of the saloon dipsticks are close to the same.

This is what I've measure from my 3.4 Mk2, first registered January 1963. It has a steel sump.. The distance from the flange to the tip is 351 mm (13.8 in). The length of the tube is difficult to measure but seems to be 125 mm (5 in). The shaded area the oil should reach is from 25 to 45 mm from the tip. As a comparison, I also measured my spare engine which is 4.2 from a series 3 XJ with an aluminium sump. The dipstick is slightly longer flange to tip, about 1 to 2 mm, but the shaded area is about the same distance further from the tip to exactly compensate. The tube length is 125 mm.

 

I posted a picture comparing my 1984 4.2 litre XJ-6 dipstick to my 1965 3.8 litre S type dipstick.

Try to find the old posting by searching "dipstick" in the Search box.

 

at this point i might be tempted to simply do an oil change and after doing your due diligence to determine how much oil your engine (plus new oil filter) takes, add the correct amount of oil and then test all your sticks in the hopes that one of them will register 'full'. or close enough to satisfy the level of accuracy you desire.

if they all fail the test, i might just put my own 'high' mark on it; remove a quart and mark it again with 'low'. then throw the quart that was removed back in and be done with it.