So you budgeted $5000 for the engine but you didnt account for the upgrades required for the HP goal. THe transmission is going to need some upgrades and will need a stall converter specificly picked for your set up. Reusing the one you have may work but could also be way too much. A 2500 stall behind a mild 350 could be close to 4000 stall with a 600hp bbc. Dont forget your transfer case. Im not a 4wd guy but I know there are some out there in the mid 80`s that would break pretty easy with stock 350`s if worked too hard.
Then you will need to upgrade the fuel system. 600hp is going to need soem fuel. I wouldnt run less than a 1/2 line with that much power. You dont want to starve it for fuel.
Then the axles. Sounds like you have a plan for those but you still got to budget for them. The ones you find may need a rebuild. Dont forget wheels as the 1 ton is going to have a different bolt pattern.
So your $5000 budget is looking pretty thin right now. Im a old school guy but even I can see the LS swap makes much more sense than trying to build a bbc these days. Look for a 6.0 or 6.2, install a cam, maybe some ported heads, get a tune and you are almost there.
If you want to go bbc still then your best bet is to do a 496 stroker. You can find a good deal on them. Id look for a small dome or even a flat top rotating assembly. Then find some 781 or 049 heads to work with. Put some 2.25/1.88 valves, bowl work, gasket match, may have to do some chamber unshrouding for the 2.25 valve but its worth it. For a camshaft I would use this,
http://www.claysmithcams.com/bbc-454...x-of-5700-rpm/
It`s proven to make strong torque in strokers and will work great in your build. Chris Straub designed them for Clay Smith. Google him is you dont know who he is.
Wont make your 600hp goal but should be damn close to 600ftlbs or torque and thats what you really need.