The 300 will take very little abuse from the big block Buick. When Buick put it behind the Nailhead in the 65-66 Gran Sport Skylark, it had extra clutches and greater line pressure to make it more durable. The one in your 68 does not have that protection. You might think about a beefed up TH350. That it will fit in place of the 300 using the same driveshaft yoke and driveshaft.
The 300 and Power Glide have little in common. Built glides were/are used in very light and powerful drag racing vehicles. The extra lower first gear of the three speed actually wastes energy and slows the very light and powerful combination down as well as uses extra energy to turn.
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