A low deck is limited to Indy MW intake. With an RB block you can run a Super Victor intake or the 3X intake.

850 hp isn't too difficult with well prepared -1 heads. I don't think you want a really big engine though, I'd probably keep the stroke at 4.15 or 4.25. Use a race block and a 4.500 bore. (or 4.440 to start with so you have some cleanup left in the block)

If the heads are done correctly then you'll just need some compression, a big cam, a good intake, big carb and good oil system. Some pan vacuum will help.

I've been making more than 900 hp with my EZ heads on the 514 shortblock, but the engine isn't very reliable. I went with fairly high compression to make the power and the margin for error is small. For a bracket engine I'd probably keep the compression down around 13:1 just so the engine isn't so twitchy. You can make more power by going with more compression but then you really have to stay on top of the tune up.