ZF box modifications help!

["Greg" <greg@xxxxxxxxxxxxxxxxxx>]

I have a non-electronic control version of the ZF 4HP22 4 speed box with
the usual lock up in 4th gear.  I have a full rebuild kit (A clutch gave
up, need to do the mod/stator/Teflon seal and going through the box
while it's out) a 2nd oil cooler to go in series after the radiator
cooler and an in line filter.  While it's out I'd very much like a
modification that will improve my use of the car a great deal as I do a
lot of gentle twisty road driving at around 50mph, including some light
towing.  I need the torque converter to lock up in 3rd in these
conditions, in 4th it only locks over 55mph and brings the engine revs
very low (I have a 3.07:1 rear I believe) meaning I'm churning oil most
of the time I'm driving and it's not the best drive.  I don't mine even
putting a valve in and having a manual override to lock the clutch up
while leaving the selector in 3, any method automatic or not would be
good if I can select lockup when rolling around the 50mph mark.

If anyone is familiar with these boxes and can share some thoughts I'd
really appreciate an email.  

Many thanks in advance,
Greg (UK)

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