Just FYI but Walbro pumps tend to not like higher fuel pressures and can fail if the pressures get too high. I've noticed in the 911 forced induction world, for some reason most people seem to be running ~5-6bar fuel pressure typically. I'm not too sure if a Walbro pump could withstand those pressure levels reliably. It all depends on how much pressure you are running, but I'd highly recommend taking a look at the specs for whatever fuel pump you consider. Most typical aftermarket pumps you'd find on the market (like an Aeromotive A1000 for example) seem to be rated at their highest flow rate around the 40-60psi range. Higher pressure equals a lower flow rate of course.
, I believe most n/a 911's (not sure on all years) have a single front pump at the tank. I know for a fact 3.2 Carreras do, and I know SC's do as well though I'm not as familiar with all of the older models' fuel systems. A single front Bosch 044 pump would be sufficient for most people as they are capable of handling up to around the 600hp level. Not to mention it would simplify things quite a bit over 2 pumps and you wouldn't have twice as many "failure" points to deal with should any issues arise in the future (ask me how I know
As for which is better, in-tank or external, I really don't know. External pumps have worked just fine on plenty of cars making tons of hp for quite some time. An in-tank pump is going to require modification to the fuel tank. An external pump would be a heck of alot easier and would still work just fine.