Well,i've looked thru the 100% completion FAQ on GameFaqs and it doesn't say that you have to deliver the Sucuricars in order to get 100%.This is all it says:
"JOEY MISSION 3: VAN HEIST
DIFFICULTY: 4/10
1.Before activating the mission, steal a police car. Then when
you actually begin you'll be in the best vehicle for the job.
2.The Securicar is cruising slowly around town. Wait until you
can hit it at top speed, and then do so. Drive away, get
another good long run up, and then charge it again. Small
knocks don't have much effect on the armoured van.
3.The police will automatically start taking an interest in you
as you smash up the van. Whenever you get two wanted stars,
leave the van and go to your hideout to get them off your back.
There's no time limit to destroy the van, and so you can do
this as many times as you need to.
4.Eventually you'll make the van stop and its passengers will
flee. If you've no wanted level, jump in.
NOTE: This is the your first good chance to find another Securicar
van for the Import/Export garage. You can't take the one in the
mission, but driving town in this one drastically increases your
chances of finding another a new one. If you don't find one after a
while, don't worry; you'll get other chances. But if you do find one
then ditch the battered Securicar and take this one to your hideout.
It can just about fit inside the garage there, and when you've
completed this mission pick it up at your leisure.
5.Take the Securicar to the docks, to the garage on the map.
NOTE: You can now take Securicar vans to this garage for a small
reward. Only do it if you've delivered one to the Import/Export
garage."
But i'm not sure.I don't remember exactly if you could just keep delivering the Securicars or if there was a limit on how much you could deliver.If there is a limit then you have to complete that aswell.If however you can keep doing it forever then you don't have to do it in order to get 100%.