Can i swap 2010 mustang v6 engine into a 1998 mustang convertable? Wondering if its possible and is it legal?

The short answer is yes. It is legal provided you keep all emissions systems that the car would have had for that year (and since they don't take them away, the 4.0L is fine). However, you will have some work to do. Having a totaled car makes it somewhat easier since you nearly all of the parts already. It's not a simple engine transfer. The engine, harness, computer(s), and transmission need to go with it. You may have to buy a custom driveshaft or have one of yours shortened since the 1998 driveshaft likely will not work on the Tremec T-5 (manual) or the 5R55S (automatic) and the 2010 driveshaft may not be the right length. In addition, because the 2010 is on the S197 platform and the 1998 is on the SN95 (Fox) platform you will need a different motor mount, K-member, and transmission support.

Thank you so much man!!

Not all emission standards are the same on every vehicle there is. If the engine your putting in your vehicle is larger than any of the other trims for that car i would use the new cars emission standards that you are pulling or buying the engine from. This would be very helpful to know if you are pulling one from a specific car or even just ordering the engine online.