what driveshaft I must use if put an IRS 2003 in my 2000 gt
None, cut / shorten or weld-in a piece to lengthen yours at a drive line shop. And they will tell you how to measure between the U joints. Re- balancing it is nessarsary.
Search Ford Mustang Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Mustang Experts