No the small block Chevvy is NOT a straight swap from a buic block!
Its the engine Overfinch used in later conversions, but due to the torque the things had they had to go to a lot of trouble to stop them breaking stuff, and ultimately, knowing that the cars they fited them to were unlikely to be used seriousely off-road in low range, didn't beef everything up as much as might be needed.
People that have used small-blocks have broken stuff, and getting one to work under hard off-road strain without breaking has normally resulted in them becoming a rebodied Chevvy Blazer!
Its a '76, presumeable three door, and apraising the variouse merits of that motor, the things that stick out are the fact that it s a two door, and rare, and slightly sought after as a 'classic', where the market preffers them to be as unmolested as possible!
Ergo, lots of reasons to leave the thing alone, or if you have to do anything to it, take it back as close as possible to original show room condition.
If you want to get silly with a mega off-roader, you can buy a tatty late eightiies or early 90's four door, that no bugger loves or cherishes, and wont raise an eyebrow if you molest it horendousely...... (saw a picture recently of one with a fibreglass replica Hummer body on top!)... for a less than a chevy adaptor plate.......
Or you could get your self a nice tidy 3.9 or later vintage, that has almost twice the power of an early carb 3.5, and costs probably half what you could get for the 'Classic' 76 model, even in a delapidated state!
Ecconomically, LEAVE THAT ONE ALONE....... as the idishman asked for directions, Start some-place else if you want to get silly building a hot-rod!