What's the budget ?

@440 CI you need 2.3 Hp/CI to equal what Brett did with the Vic heads, @400 CI you need 2.5 Hp/CI
Apache heads will go over 400CFM with a 2.20X7MM valve and port work,but I think your better off trying with the ThiTek heads
I think Rockers are the only obstacle to over come(besides the cash to make it all work)
For lifters I'd use .937 keyway, or maybe .904 dog bone, (MY OPINION) your not going to achieve this goal with .842 DIA lifters,
it would have to have external oil pump.
6.1 block needs Darton sleeves to keep the cylinders tight at the bore size you need to make 1000HP NA. Bore needs to be 4.125-4.15 maybe 4.16.
BGE block ..from the sonic #s I've seen could mathematically go to 4.185. that would leave .120 as the thinnest wall , good thing there is it's never the thrust direction and it's usally cylinder no 8 at the end of the water jacket.
The added deck height of the BGE makes it the better Iron block candidate.
4.05 crank would max stroke even then your looking at a RR 1.53 with a 6.2 rod and a Pin height in the 1.04-1.05 range..
Steff's G3 is good one to look at as a starter, I think he made close to 800 at 400Ci (not 100% on that) need him or Ray (SIXPAKGUT) to step in and correct me.
Buddha boy's 449 Alum is almost 2 Hp/In I think he made 890HP @449 CI
HRH Performance has done 840/850HP @ 427CI with an Iron block and ThiTek heads...and according to them 6.1 lifters.
I KNOW a G3 can get close , I'd love to see what Brett could do with a set of Apache heads......is it Brett or Bret ?