I am 90% completed with my K5 swap into a 01 KX250 frame. I used the heat gun/ BFH (big fing hammer) method. I ended up burning a hole into the desert tank, but it is rather easy to heat up the surrounding areas to "Weld" it back together. On my 3.1 or 3.2 IMS desert tank, you have to massage 3 places:
both sides where it hugs the head, and then right above the spark plug you have to completely manipulate that area to get it to fit. I have to put the tank under the heat gun again to get it to completely clear the shorty plug I'm using ~ but I'm close.