As far as nitrous goes keep in mind a few things.
The 2 bolt main cast crank 11/32 rod bolt standard rods with pressed in pins and cast pistons never made more than 300HP and were redlined at about 5200 RPM in a factory build.
The 4 bolt forged crank 3/8 bolt rods with better pistons were rated at a max 370 HP though I think that was underrated 400 HP being more accurate. This set up red lined at 6000 to 6200 RPM (higher on the shorter stroke 302 and 327) in a factory build.
Of course hot rodders have been getting better numbers than GM did with many tricks and improvements.
Such as: Chamfered oil holes, cross drilled oil passages, main bearing studs, higher volume and higher pressure oil pumps, modified oil pumps to reduce pulsing, better harmonic dampers, hypereutectic or forged pistons, better piston rings, windage trays higher tensile strength rod bolts and of course all new valve train.
SB 400 never came with forged cranks they were lower RPM torque engines and were very heavy duty, you lost some of that heavy duty turning down the bearings to fit the 350 block and rods.
The stroked 383 using factory rod bolts had to have one outside edge of the bolt head ground down for cam clearance these usually lasted 50,000 miles or more but you could see the bolt head failing somewhere above that number. (That number will be lower pushing it with nitrous.)
If you ever tear it down measure the main caps to see if they are egg shaped (the top to bottom longer than side to side this will tell you the caps were being over stressed and needed studs or aftermarket parts preferably 4 bolt splayed forged steel caps with the 2 straight bolts using studs. Same can be done with the rod big end the only cure is aftermarket rods.
Studs on the main caps are good for about 50 more HP and 300 More RPMs on a 2 bolt, 100+ HP and 500+ RPMs on a four bolt.
If you really want to gauge how far you can push it with nitrous you need to tear it down and see what was really done to strengthen it up and look for signs of over stress from what you are running now.
All numbers are off the top of my head and may not be 100% accurate, but they illustrate the idea.