I would go with the 383 since they are good engines where a 350 is to weak and a 454 is to big. just make sure you find a block with four bolt mains not the regular 2 bolts. The extra torque the 383 makes breaks the main caps off and throws the crank. I have seen the aftermath of that in a 80's camero that was converted to a 383 on a two bolt main what a mess.
As for diesel. Easiest would be to go to a 6.2 or a 6.5 turbo chev block as they mount to the same trans as a 350 and the basic computer of the 6.5 turbo would be a easy swap for you.
The Isuzu engine is good until you need injection work and then they never stop leaking and if you have bio diesel, don't run it cause it will mess up your injectors. Navistar engines weigh a great deal more then the GMC diesels and the Cummins, Also the related recals on the 7.3l and a 6.0l isn't worth anyones time since they blow turbos like kids eat candy. the Cummins ISB 5.9L engine is used in everything from the Dodge Pick up to some class 7 trucks so parts are easy to get.
Cost has to be taken into consideration a Cummins diesel from a wreaker can cost around $6000-8000 a navistar about $4500-7000 a GMC well that varies region to region but they can still go for around $5000. Correct my pricing if I am wrong it has been a while since I looked at smaller diesel pricing.
