400 budget compression HELP +++++
I agree with Mr. IQ52 in that you have a lot of milling going on and it will a pain to figure out how to line all that stuff up once it's done. I can see elongating intake bolt holes and trying to figure out how to seal the valley tray in your future. Make sure that head bolt holes are tapped all the way down.
A 75 CC chamber could be tough to work with on a zero deck engine that runs on pump gas. Be careful with piston-to-valve clearance etc., you will need to get the clay out and measure carefully. While not huge, that cam you linked to is still pretty big, so you will have to really be certain that you have enough room. The pistons only have a 5cc dish. You have to degree it for certain and run it through a few revolutions to make sure its right.
Not sure how much you have in the heads but it's always a tough call to put money into old, factory pig iron parts when there are shiny aluminum heads available for decent money. But, you are already into the heads now for what seems like a bunch of coin so anything 'budget' now is going to have to be made to work with your heads.
Looks like your budget will consist of pistons (pressed or floating pins?), rings (make sure the current bore finish will work with whatever rings KB specs), balancing, (may require Mallory metal since the KB pistons are lighter) deck height measurement and surfacing, head gaskets etc.
I still believe building the best short block you can will yield the best results for now and for whatever future plans you have for the engine.
