Will a t5 from 1987 mustang fit in 1998 mustang?

No, a T5 transmission from a 1987 Mustang will not directly fit into a 1998 Mustang. While both cars used T5 transmissions, the transmission itself, bellhousing bolt pattern, and input shaft are different between the Fox-body (1987) and SN95 (1998) platforms. You'd need significant modifications, potentially including a new bellhousing and possibly even custom driveshaft work, to make it fit. It's not a straightforward swap.