I would save that Buick/2 carb & contact the Buick Club for a buyer, making a profit in the process.
At the same time; find a 261 for your truck. It "bolts in" and looks original to most, making the truck gain in value every year, forever. Plus; 60 more horsepower (stock).
To do it otherwise is not as good a 'financial bet' as; a rare combination like that will be very hard to sell later. It will require a lot of conversion/extra money, right now too.
What transmission could you use??
Many "shy away" from engine conversions on early vehicles because of this. There 'harder' to re-sell later & make any money on. Often there's so many problems involved there never finished and 'all is lost' (money-time wise).
Please do some research before you lite the torch.
Patrick's number is: (520) 836-1117.
Good luck.