Coach,
For a boost gauge most people run a line from the intake manifold. There should be a hollow bolt/banjo fitting at the back of the manifold with a line running to the ALDA on top of the IP.
T-ing into this line will be fine for a boost gauage. For your LP injection you might want to consider asking the engineer his opinion on introducing the LP before the turbo. In the picture the guy has a pressure regulator in the line to the waste gate.

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Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog