My stock gauges work fine. I didn't use the stock speedo but you can make that work too. The oil pressure just hook up the LSx sender to the '81 gauge. For the temp get a 3 wire sensor and plug. Two of the wires will run the computer and the third will go to your '81 temp gauge. It is kind of pricy but there is a box that takes a speed sensor signal and turns the speedometer cable. The weather is getting a little bit cooler now so I need to get the engine back in the '81. Then it will be for sale.
2006 Superior Blue Trailblazer SS AWD, Stock as a rock
Sold: 2002 GMC Sportside Denali front end with a 2002 LS1, FLT level 5, Yank 2600, Trick Turbo, T76, Nelson intercooler, 60LB injectors, Warbro fuel pump, Eaton locker, Magnaflow, 3.42, Nelson Performance Tuning (speed density).
Sold: 1981 GMC LB RC 1500 2002 4.8L 4L60E 12 bolt 3.73 Richmond Lock Right AC PS Nelson Performance tuning