If you peruse the Jolly GMC site you’ll get a collectively strong opinion that the GMC V6 was the best engine ever built. If you buy a GMC for restoration I would highly recommend joining:
http://www.6066gmcguy.org/From a durability standpoint I would agree. The crankshaft is far up in the engine block compared to just about anything else. Everything is over-engineered and built to run millions of miles. It is rugged and a torque monster. For pulling a big rig or a house or tree stumps it’s in a league of its own.
From a practical standpoint for a daily driver, I believe the V6 weighs over 850 pounds, and is sitting right on top of the steering linkage, which certainly doesn’t make steering any easier. They get very poor mileage, parts are rare, and finding a person willing or experienced to rebuild one can be a challenge. A rebuild will probably cost over $3,000 as well. There is a common swap adding a Holley two barrel carburetor for somewhat better mileage and ease of tuning, and electronic ignition can be added. There may be other upgrades I’m unaware of, but those are about the only performance enhancements I know of available for this engine series.
By comparison the I6 is much lighter, parts are readily available. High performance and cosmetic equipment is produced should you want it to really stand out. But these parts are not widely popular (compared to a V8) so the cost is higher than for a V8.
There is a reason the small block V8 is so popular and common. There are unlimited and reasonably inexpensive ways to build this engine for mileage or performance. It’s light and small and easy to work on. Parts are available nearly everywhere, including aftermarket components for just about any engine accessory, which really keeps the cost down the farther from stock you stray.
For a daily driver I would recommend either the V8 or inline 6, based mostly on personal preference. I am a fan of the I6, although I have not owned one in many decades. It turns out I enjoy the convenience, performance, and practicality of the V8 even more. I would only recommend the GMC V6 if you want to own/drive a restored GMC, or truly need the torque they can produce.
