The intake might be a problem.
In 66 they changed the angle and size of the intake manifold bolts
So a 65 and older while the same dimensionally will have issues bolting onto the 69 heads.
I understand the intake can be machined to work but for the money you might want to either get 65 and older heads or a 66 and newer intake.
Also I applaud you for going 273, but a 318 would be much cheaper, the pistons on a 273 are in the grand range alone.