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Had the chance to test the GMC on a chassis dyno today.

Did two pulls:
1. Propane. 123.7 net HP (DIN norm) at 3500. 296 Nm (219 ft lbs) at 2380 rpm.
2. Gasoline. 145.5 net HP (DIN norm) at 3990. 312 Nm (230 ft lbs) at 2480 rpm.

This is a good running but worn 305E with 125 psi in all cylinders, a very worn timing chain, a 2.25-inch true dual exhaust, HEI ignition, and the stock Stromberg WW carburetor.
SM420 transmission, 4.10 geared Dana 60 on 235/85R16 tires.

As you can see, the torque curve is fairly flat, with torque peak at 2400-2500 rpm compared to the factory's 1600 rpm. We tested to around 4000 rpm.
It also lacks some torque compared to Jolly's chart.

I am assuming the 305E that is in this truck is a fairly worn engine, abused throughout the years. Combined with the wide ratio SM420 it isn't a fast truck by any stretch of the imagination. I don't mean to race the thing either but I do like some power

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The power losses with running propane are definitely noticeable and this can be seen in the results.

I intend to gain a lot more power with the 478, and then I'm planning to repeat the chassis dyno test. very curious to see how it will perform.