This^^^^  

Get a  gasket kit,   as   a lot of  things  need  swapping;   oil pan,   water manifold,   water pump (at least if was  on my  4450)    Exhaust and intake manifolds.     IIRC..  front harmonic balancer.

the  'plug' in  the end of the  combine's  crankshaft  needs to be pulled out for the tractor  flywheel to  fit.  

Might as  well   pop in new  front and rear  main seals.  

Basically,  you take it down to  short block then  assemble it with  'tractor'  components.  

Injection pump;    either  swap the tractor in place,  or  have a   pump shop   reverse the controls on the combines.  (everything is  backwards)   IIRC,  but I've done  several  Perkins/massey swaps too.    I may be  wrong  with the JD  pump.

I've  got  almost as  many hours on  my  'swap'  as the original   tractor had.        I don't  care  what  'brand' of  rebuild  kit  is out there,  or  who assembled it.     Original   parts  simply  out last   ANY   rebuilds  today.  

With  'core'     you  probably get an engine  shipped in  for    3500-5000.     I keep the cores,  as   having  a  spare  (whatever)  laying in the   shop is  worth  far more than  the core/shipping it back.