I know this problem of too many half restored classics on the premises. It's a combination of a) late payers b) having too much work on the go at once.
A is solved by agreeing with the customer, in advance of the work starting, a staged written restoration plan and staged costs eg £xxx on strip down. £xxx on welding completed. £xxx after paintwork finished etc. Then both sides know where they are on the plan. After each stage invite them in to look at your lovely work, and pay you.
B it's tempting to take on a lot of work at once, but it can come back to bite you.
I think its better (and less stressful) to concentrate more or less on one vehicle from start to finish, then you have a happy customer who pays the final balance and leaves, spreading the good word. Then ring the next customer up to tell them to bring their van in. If you cant start work on their van until August or whatever, once you have built up a good reputation people will wait.
Cheers
Jules
RatSport www.ratsport.co.uk