Lots of 510 guys install 280ZX front struts on their 510s to upgrade the brakes and get a shorter strut and more available choices of inserts.
My last 510 had a set of 280ZX front struts. I had to remove the lower spring perch then grind down the area where the weld was and then installed a set of coilovers from Dave Turner Motorsports. I didn't have access to a welder so I bought a set of clamp on collars from Dave turner that the bottom of the threaded collar rests on.
Ground control along with some of the eBay cheapy coil over kits will work fine too.
(hacksaw or grinder will work)
when you see rust in the cut you are through the perch. Don't cut into the tube.
then free the perch.
All these photos are from a 510 guru (Kelvin) see his entire photo section on strut shortening at:
http://www.datsun510.com/photopost/showgallery.php?cat=3098&ppuser=9
If you don't have access to a welder Dave Turner sells a clamp on lower collar that the threaded collors will sit on. Sorry, no photo but i used it for several years with no problems. One nice thing about this is you can adjust the lowest setting while on the car and get the right stance without having to tack it, see if it right, cut the tack, adjsut it again, etc.
If you want to go even cheaper, get a set of muffler clamps and throw away the u-bolts and buy some straight bolts and nuts that are long enough to go all the way through them.
Before you all start bashing this as unsafe, many 510s and other cars have used this method for many years with track days, autocrosses and street use without failure. If you are worried, don't do it.
The Dime Quarterly did a product review of the Dave Turner do it yourself Coilover conversion. The review will soon be available on their website under product reviews. I have the actual article in one of my back issues. It is a well written article and has pics of the set up and also shows the clamp on collar witch is much better looking then a set of muffler clamps. I can fax it to anyone that wants to see it. PM me.
Oh yeah, the Dave Turner coil over kit was about $200 for both front struts and about $30 for the clamps.