The wr450 isn't a "swap". It's a complete custom build. You end up with a dependable modern 4 stroke with electric start and 12" of nice cushy modern suspension front and rear, and more HP than a built 350x or a stock 250r.
It's easy to make more HP with a 250R, not so much for the 350x. There are some cheap things you can do with the 350x that will help it run better and make a bit more torque, but to get them to make a bunch more HP is expensive.
YAMAHA 450 HYBRID
85 350X- RED
85 350x -BLACK
86 350x-WHITE (with Goki)
85 250r
83 atc 70
84 atc 70
84 atc 110
09 yfz 450
2006 Arctic Cat Prowler
RZR XP 900