there is a cover/plate on the side of the oil pan, above the oil filter, held on with 2 bolts ,called the oil cooler block off plate, that can be used to tie in an oil pressure fitting to supply a guage. the plate is easily removed, drilled and tapped for whatever thread you want (within reason). a small copper line would be better than a plastic line in this case because the plastic may end up touching the exhaust. fire hazard, leak to clean up at least.
check this link for a pic of the oil pan with the cover attached above the filter. mayber easier than getting to your oil sender behind the intake manifold. there are also predrilled/tapped covers sold as well
https://ls1tech.com/forums/conversio...-oil-pans.html
https://paceperformance.com/i-814627...eaded-cap.html
https://ls1tech.com/forums/generatio...off-plate.html
http://67-72chevytrucks.com/vboard/s...d.php?t=493630