I'm running 8-10lbs of boost daily on completely stock internals. The only significant problem I've had in over 2 years is a leaky headgasket, and that didn't happen until a couple months ago, and was easily dealt with. I would say if you can run up to 8lbs of boost with an excellent safety margin, and a bit more if you're confident in your tuning. Some people run 12lbs of boost daily on stock D series blocks, but stock rods are like toothpicks, so I would say that would be the max I would run on a daily driven stock D block, and only if your tuning is up to snuff.

The cheapest way to get enough fuel for 8lbs of boost is to use a 12:1 FMU and an inline pump. You can get them for less than $300, total, and they'll allow you to run 8, maybe 10lbs of boost on stock injectors. The reason I cap the boost level at 10lbs with a 12:1FMU setup is because of the extremely high fuel pressures that they produce under boost. Even at 8lbs of boost, if your static FP is 40psi, your FP under boost is going to be 136psi, which is already pretty high. If you raise the boost level beyond 8lbs, your FP is going to go up 12psi per lb of boost, and it doesn't take long for FP to go extremely high.

A much better, although slightly more costly fuel setup would be an AFC and some 450cc DSM injectors. An AFC is around $300,a nd DSM 450s can be had for ~$50. With the AFC set at 40% fuel trim, you'll be able to run and idle the 450s with no problem, and the decrease in the MAP signal also allows you to run up to 10lbs of boost without the need of any check valves, and you won't throw a MAP code. You'll need to wire in a resistor pack, or inline resistors into the injector harness, but that's not very complicated. With this setup, you won't have skyhigh FP's under boost, either, and you have room to upgrade down the line. If you decide to step up to that AEM you have your eyes on, the 450s will work with the EMS, and you can sell off your AFC.