I bought this kit awhile ago which NOS makes for both squarebore or spreadbore carbs using a plate with WOT switch, 10lb bottle, lines, two solenoids and 6 total jets for three power levels of 100, 125 and 150HP.

The engine:
350ci SBC 0.020 over, all else was good except for a water jacket crack repair
original 882 78cc heads, standard valve job with cam matching springs installed
new oil pump
holley 3310s 750cfm vac. sec. manual choke
holley mechanical fuel pump (flow for 520HP)
1/2" custom bent aluminum fuel line from tank
holley (weiand) stealth intake (more flow than edel. RPM) 1500-7000RPM
comp cams 275DE (219/229; .468/.488; 110)
original lifters (all perfectly straight)
new camshaft chain set
original crankshaft all machined 0.010, endplay checked, magnaflux + UV
original rods weight matched (for free) to crank and pistons
silvolite cast flattop giving 9.25:1CR
holley (flowtech) "afterburner" full tube headers 1-5/8" pri, 3" collectors
flowmaster 2.5" dual exhaust
new HEI distributor
accel recurve kit
accel HV HEI coil
accel 8mm wires
highgrade dist. cap
original harmonic balancer (great shape for a '74)
summit 2 core 1" tube aluminum racing radiator
The engine idles well at 900RPM, runs at 600 but doesn't rev fast enough from there. 1 hour of engine running time, fuel, oil, water, voltage levels excellent, runs smooth up to 5500RPM (haven't tried taking it higher just yet)

Now for my question: What power level is safe to use for this engine once it is fully broken in?

 

I have only personally been associated with one person using a mechanical fuel pump with nitrous. He ran a 50 shot and had no problems. Your cast pistons are going to be equally as vunerable as a mechanical fuel pump. I don't believe you mentioned what kind of rings you used. A typical nitrous engine has forged pistons with moly rings (people do well with small shots on hypereutectic pistons also). Cams with more lobe seperation respond better to nitrous (110 isn't bad, but 112 and up seem to work better) as well as single plane intakes. I've been told that a 150-200 shot is about the limit of a dual plane intake before "weird things happen". A 150 shot with my Perf RPM worked fine.

I guess I can't put a number on your engine, but I would say none with a mechanical pump and cast pistons. Just my opinion though, I'm sure someone will disagree.

 

My friend had an 86 Monte SS with a completely stock ZZ4 crate motor with hyper pistons and a mechanical high-volume fuel pump. He sprayed anywhere from 100 to 150 shot all the time with no problems. He had a fuel pressure gauge mounted to the hood so he could keep tabs on pressure, but it was fine. I'd say a 100-125 shot would be fine all day long. Just keep the tune slightly rich, and retard the timing about 4 degrees or so. Better safe than sorry. Good luck!

 

With nitrous I am a fan of electric fuel pump from a more steady flow of fuel.

 

I've got moly rings in it. I'll have a window switch set at 3000-6000 RPM, of course using a full throttle switch (at the carb not pedal) and a shifter switch.Going to add a fuel pressure gauge as well but I've already calculated the amount of fuel required at full throttle at 6000 RPM at full load and the 110gph pump gives me a good 10% margin. Its just pistons/rods/crank I'm concerned about. The engine is connected to a 3 speed auto so I know the converter is going to absorb several forms of abuse. I'm going to do this the safest way possible and start with a 50 shot, then move from there. Thanks for the responses.

 

With nitrous, they always save start low and work your way up. I would get all the bugs worked out first before spraying.

 

You should be fine up to 150 I would add a FPSS just to be safe. Plenty of people run plate kits on mechanical pumps and while it is not ideal it does work. The biggest problem you may run into is tunning it since the nitrous and carb usually like diffrent pressures.