[AZKen]

Not sure the probe will fit, may hit back wall. That plug is a drain I believe. I don't think a backup mechanical gauge is necessary. If you must have one, it should be near the electric one on the front water crossover manifold. A Chevy thermostat housing (water outlet) with a pipe threaded off-set hole is available. May have to enlarge one mounting hole to fit V6 pattern, not sure V6 T-stat fits this housing but try Murray 84842 type..... Or you can drill and pipe tap into the crossover manifold, front of manifold, driver side (large flat area). Mount probe horizontal so as to clear T-stat. Either way will keep the two senders (switches) near each other for accurate comparison.