Go check out a builders supply yard, grab a couple of bits of threaded scaffolding tube, a couple collars and a couple of lock rings.
Before removing, rest it on tye ground, and meadure distance between the top and bottom spring perch seats.
Rip your struts out,measure uncompressed free length, same perch to perch, theres your static compression height.
No pull struts apart, carefully grind perch off strut, slip said threaded acaffolding tube over, tack weld, fit lcking ring, then collar, machine disc to suit ID of springs, reassemble, using current strut top and perch.
PRESTO!
Coil-overs, then start playing with ride heights.
Start from there, then worry about shortening strut, thicker wire diam springs, and shorter length strut inserts, later
( Toyota MR2 are nice and short!)
You should know all this simple stuff,,,,,,
Crikey,,, I just noticed,,,, 3900 posts on here, of usefull knowledge, information and years of experience on lots of things Datsun!