ISX or Signature 600 transplant

My son is looking to urchase a 81 pete 359 exhd with a KTA 600 in it. This truck is a one owner one driver truck and except for the engine being tired and out dated is in awis good of shape today as it left the factory in. Now he's wanting to repower it and update the transmission But is wanting to keep it cummins and is thinking of either a Signature 600 or a pre EGR ISX with some tuning done to it of course. My question is what is the difference between the two engines. is there a displacement ddiffernce or just the electronics different? Seems I heard of the early Sig600's having connecting rod issues. We had one of the later ones that ran really good and got decent fuel mileage but always ran high oil temp. So being that I can't talk him into going with a cat which engine would be the better choice? We know where a running core Sig600 is that was replaced with a cat when it got tired is so locating one is not an issue.

 

Block is shot on the KTA. Engine has over 3 million total miles on it. Last time it was inframed cummins told us it should be done then. Engine still runs but loses water into the crankcase. Block has been sonic checked and determined to have pretty severe core shift. As it sits right now there's an exporter that will give $8000 for it being it still has factory spec crank. If all my son was gonna use it for was around here local for the farm id probalby keep the KTA and try to find another block. But he wants to go out on the road with it so it needs upgraded. I wanted him to put a cat in it but he's keeping it cummins. His money not mine. I can recall a lot worse things i spent my money on when i was his age. He's only 20.