I like the RTE setup. Really a better design, but $50.

The linear regulator I pictured, is mounted inside the original IVR case, and the heat sink bolted to the outside of the case, with heat sink grease between the regulator, case, and heat sink. Not the ideal configuration, but with the stock case it plugs in like the original IVR (no soldering to the instrument panel.)
The regulator I pictured is a slightly better 1.5 Amp 7805 regulator, but that was many years ago. I haven't looked recently if there are even better versions now?

I think on my '69 rallye dash, each gauge pulled about 1/4 amp at full scale. Without the heat sink, at 3/4 amps (worst case - full tank of fuel, maxed oil pressure and water temps), the regulator will start to go into thermal shutdown (reduced voltage output) after about 10-minutes (depends on ambient temp.)
At 1/2 amp, I did not see any voltage drop without the heat sink, even after several hours. I think if you just have fuel and water gauges, just using the stock IVR case as a heat sink should be OK, but with all three gauges, the additional heat sink may be needed? I did not have a way to increase the ambient air temps when testing, and I din't know how hot it gets under the dash in the summer time?

On the other hand I almost never have a full tank of fuel, 100 psi oil pressure and 260 F water temps, so the real load is much less than the 3/4 Amp max, but the regulator needs to handle that situation to make sure the gauges don't read low when they should be reading high.

Really, if you have the money, just get the RTE regulator. If on a budget, the 7805 setup works OK but know its limitations.

If you need the 7805 regulator and caps, I still have some extras.