r3 street usage

If there was a block that would hold your power goals, the R3 is the one. The magazines, etc. claim that the 340 replacement block can only to .080 overbore. A used X block is another possibility.

The R3 should be more than enough adequate for street performance. I am not sure if all R3 blocks have motor mounts or if some require motor plates. The only functional issues that you might develop, could spawn from overheating issues if you run a Siamese bore block (up to 4.22 bores advertised) that does not possess space in between the cylinders for coolant. Chevy guys manage to deal with this issue with their 400 small blocks. W2, W5, Brodix, or Indy cylinder heads would be required for that performance level. I think a 600hp street engine might be more difficult to build than race purposed engine would. If I remember correctly, a new block often needs significant machining of the deck on the R3 engines.

Brian & Fred Hafliger at IMM engines www.immengines.com/

Mike at http://www.mrlperformance.com/

and Ryan Johnson at http://www.shadydellspeedshop.com/

all have the answers, blocks(?), & experience that you want.

I am not an experienced professional, but I hope this points you in the right direction. Research & plan it out, and good luck!