From what I can see , I think they are a little too close . Maybe another 4" to 6" further back and if you can , have them at least at 45 deg angle . Your call .
You need to put the O2 6" to 8" after the collector so it can read more accurate . Make sure the O2 is facing up , less chance of getting water ( moisture ) contamination ( inside exhaust system ) .
Headers look great . Place your O2 bung about 6" to 8" after the collector . So you have to remove the motor to get the headers off ? And what about starter replacement , header comes off ?
V8 anyone ? Don't get me wrong , I like Nissan motors except for my Z32 ( a pain in the a** to work on ) . An LS motor will get your HP goal with ease and it's lighter too . As for the cost , if you do it right you can have it for far less than 6K .
IMO , Gen I and II Chevy are old tech . Most people are converting old Chevy small blocks to LS series motors . If you are planning on a V8 conversion , go with the LS . It'll save you time and money in the long run .