You will be fine with the 1.70mm, you will have plenty of clearance, you really have to determin your CR by measuring everything using the burrette and fluid to get accurate CC readings. If you need higher CR then you need to mod the heads, order different pistons, shorten your rods, etc.
I assume you have your heads machined for twin plug? That adds a .5 CC or +/- to your head what else was done?
Most stock 78-79SC heads that have been discussed here are measuring out to 87.5 - 90 CCs.
This thread explains it a little better:
http://forums.pelicanparts.com/911-engine-rebuilding-forum/818505-why-my-deck-height-2mm-what-can-i-do-reduce.html
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78’ SC 911 Targa - 3.2SS, PMO 46, M&K 2/2 1 5/8” HEADERS, 123 DIST, PORTERFIELD R4-S PADS, KR75 CAMS, REBEL RACING BUSHINGS, KONI CLASSICS
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