Farm all,
  Around 1955 or so, I was pretty young but my Dad and brother took an F-12 and installed a Ford flathead V-8.  They took off the front steering casting, flipped it in the rails and hacksawed off the top.  I do recall taking my turn on that sawing job.  Welded on the original wide solid axle and car tires.  Used the rest of the steering linkage to replace the tractor one.  We kept the engine, clutch and transmission intact, adding a universal coupling to the shaft sticking out of the tractor transmission so that match-up was pretty easy.  We had an old Fordson radiator so that got stuck on front.  
  This was one ugly tractor.  Had more slow speeds than a turtle but with the extra engine revs, it would travel fast enough to get the old knobby tires bouncing.  I remember the brakes just plain sucked.  But...the dual straight pipes barked beautifully and made it all worthwhile.
  For some reason, it became the tractor we used on our Allis Chalmers Roto-baler for years, making 10-12,000 bales a year.  I guess it served it's purpose.  Later on, it and much other equipment was destroyed in a fire.
  I'd do it again in a heartbeat, but make it more graceful.  Come to think of it, some day maybe I'll describe how I mounted a big honkin' Mercury V-8 crosswise on the front bumper of an Army 6x6, cut and welded down the drive train to run a Ford 612 chopper mounted on a couple I-beams with hydraulic cylinders out front and filled the unloading box on the rear of the truck.  It's the sort of thing you do when you don't have the money to buy a large enough tractor to run a decent chopper.  Spent a lot of long nights in an unheated shop sitting' on a five gallon bucket with a yardstick figuring out how to make it all fit.  All I had was an old Forney 180 amp welder, torch and hand tools.  And some stupid determination, I guess.
  So, sure--do it and have a ball!  Many of us would like to hear your rig stand up and bark.  <grin>  Best of luck on whatever you decide. And to answer your question directly, if you're enjoying it, it's really easy.  Stetts