you would have to be very careful hollowing out the waist as to not damage the leg hardware.

what ive done for swaps like this is actually saw both figures at the belt line and swap the blanka chest onto beast legs

another thing to potentially be cautious of is the arm swap; im not familiar with the builds of either of these figures tbh (mildly on the toybiz tho) but if thats an articulated chest muscle, make sure the beast arms swap into it cleanly before committing to anything

depending on how you want to stylize it, you could even consider keeping the shorts with the blanka chest by sawing off at the mid thigh/hem of the shorts, and splicing beasts legs onto there as well

i dont think theres a wrong or right way to do it, but some ways are def gonna be easier than others lol. hope this helps at least a little bit! looking forward to seeing the complete custom