Squarenutz. I have the same thing.j
You can drill the holes for the 3 point 49 cab mount. No problem. The holes need to be oblong. The holes at the bottom of your 53 frame will allow you to get to the center mount of the cab. Unfortunately, I found that the 49 cab fit rigid on the frame in the conversion and I have the 48 frame so I am going with the 48 frame that the 48 cab came off of. If you cannot do what I have done, since you don't have the 49 frame you shouldn't have any problem modifying the mounting bolt holes on your 53 frame to make it all work.
I hope this helps.
