Allstate eventually came through on my heavily modified '90 TT after the garage fire. It took five months and a pricey appraisal by a known name in the business but it worked. I have American Family now and have agreed replacement value on my boat and Ferrari, they may do it for your Z. My new TT is still basically stock so I haven't pursued an agreed value policy yet.
I had to go with State Farm for a policy on my Champ Car, weird that they could do what AF couldn't but it all depends on the underwriters.