I agree, go with the 3psi spring, itll leave you room to go down if your fuel cant handle 5-7 and will give you the ability to fine tune.
Oil return can be in the oil pan, just make sure its above the full oil level or else it will cause draining problems.
Alot of people swear by the block return, but i dont like the idea of drilling a hole in my block, although this way makes leaking improbable even without weld.
Unless you plan on running a huge amount of boost, you wont see any losses in turbulence in routing the wastegate gasses. The extra amount of noise with a wastegate dump isnt the most appealing sound & may not be your cup of tea, and it may get some "unwanted attention"
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