You should have gone a Series 2 engine. Only reason I say this is the radiator piping is a pain on the old engine. On the Series 2 the thermostat housing is at the front.
Whilst you'll cop a lot of flack (Crappy pushrod design!!), I always like to see a different conversion. I think you'll be very suprised with the resluts though. The Buicks are very pokey down low and pull well, they do run out of breath up high.
Are you running a T-700 or T5?
I'd probably suggest a bigger radiator, and you look a little space restricted for a thermofan and shroud.