bolt up 99 block with my 98 parts

Yes....a '96 T-Bird uses a Romeo Block, which is what your '04 PI heads are. Your '98 Headers will fit both Windsor and Romeo blocks, since the exhaust bolt pattern remains the same on both. You’ll retain the same 10.4:1 CR you would had you stuck with your ’98 Mustang block. The only major difference between the Windsor and Romeo short blocks are the use of Dowel Pins vs. Jack screws that thread into the sides of the main caps. This isn’t even going to be an issue for you assuming you’re transferring the complete short block over form the T-Bird and only changing the top end and accessory components.