No, the union would only insure you obtain what is contractually spelled out by the agreement they have with the employer for your job classification. there is no negotiating here. the good from this is you know you won't be cheated of anything but the bad is your progression will not be the same in a company not bound to strict rules of progression. So you choose, there is no negative to entering into such an environment but realize the employer's hands are tied too

to be honest, if the pay and rewards of such value you really should pay for a few hours of a lawyers advice to review the contracts offered you. if the prospective employer does not permit that then that should be a signal as clear as day what to do.

only you can state what level of compensation is worth the money spend on a lawyer to read it over for you and you should weigh the time you spend doing the research against someone who knows contract law.