you will also need a metal detector. The gland will bury itself quite deep, there is pretty much zero chance of spotting it by eye.
I've been 6 feet away from a lead glande landing on bare earth and even then I couldn't find it. They go deep !
As far as technique goes - whatever you're most comfortable with.
But you will probably need a 45-50 inch sling, small leather pouch and a helluva lot of practice with long sling techniques
The two best distance techniques (based on results by their perpetrators) are yureks and larry brays.
Both use very long slings but two completely different techniques.
Yurek favours a run up and a slanted helicopter technique and larry bray uses a stationary technique based on baseball pitching. Check out the wyoming slinggolf video for larrys style and ther are a few of yurek around on youtube as well.
Larry bray has the official traditional sling and stone world record and yurek has the current unofficial slinging.org record.
Both well over 400 metres.
WE can't record an official record if the glande is not recovered.
Basically the hardest part of this record is always going to be recovering the missile. They hit end on spinning and both drilling action and impact force drive them deep into the ground.
If possible a tiny gps chip and thin wire aerial could be embedded in the glande. Other than that a metal detector is your best bet, with maybe some thin copper wire wound into a groove in the glande to boost it's signature to a detector
Also keep the glande around 2-3 ounces for best density and weight to air resistance ratio. And of a longer slimmer shape - again for lower air resistance.
And mark both your throwing position and the glande landing with accurate gps (preferably all on video) and at least 2 credible witnesses
I'd really like an official slinging.org lead glande distance record - sounds like you're well positioned to set our first one
Good luck