Brian,
Engagement of first relies in a positive engagement of the sliding gear on the layshaft with the dogs of the first gear. If there is shuddering and or chattering then either that engagement is not good OR there is an issue with the clutch.
I assume you have removed the clutch and washed all in solvent (petrol works) to get rid of the bulk of any ATF that may have been used and replaced it with a proper transmission oil such as MOTUL TransOil 10-30.
When you had the box appart did you examine all of the components of the selector mech? The bit that actually moved those sliding gears. If there is wear there then the engagement will be compromised.
Check the end float on the camshaft - it should be 0.002" but will increase with wear - you can shim it with a hardened washer as a temp fix, real fix is to replace the bushes it runs in.. also check the selector forks;' that engage the sliding gear, any sign of ridges on them means they are on the way out and need replacing. Also remove the split pins that hold the engaging pins in place and examine the engaging pins - again any sign of wear - replace them.
As you will have already found there can be hydraulic lock as you try to set up the gearbox and for that reason
all end float measurements must be done with the box completely free of grease or oil.
Just an after thought - are the primary chain adjuster bolts - 2 of them - under the gearbox firmy secured? If no it may just be that the box itself is moving back n forth as you select first.
Don't feel disheartened - It took me 3 attempts of strip clean measure and reassemble before I sorted my box - and I found working with it in a large baking dish (to contain grease and small parts ) a real boon during the strip down stage.
Martyn