[68Timber]

Several others have mentioned the cam, I'm wondering if it's wiped a lobe myself. Just pull the valve covers and remove power to the HEI so it won't start. Have someone turn it over while you watch the rockers and see if you notice a lazy one.

Also, don't rule out the carb just yet. If you know of one you can borrow off a good running truck, bolt it on and try it out.

One other thing you can check by hand or with an infrared thermometer (if you have one or can borrow one/rent one) would be to start the engine dead cold and check the exhaust port temps either by hand or by infrared thermometer and see if you have one not warming up at the same rate of the others.