I have spied a very cheap '62 327 shortblock on craiglist and wonder if it will slot into my third gen easily with the addition of modern heads.

 

Should be fine. The older engine will have the two- piece rear main seal and matching larger crankshaft flange, so you will have to find a flywheel/ring gear that will work for you, one without the small external balance weight that one piece seal cranks have.
Not sure if you would have intake manifold- to- block issues if using fuel injection. The really old blocks were designed for a breather on the front of the intake manifold by the T- stat opening. A lot of aftermarket carb intakes still have a round boss cast into them in this area that could be milled out for the old breather tube and cap.
And I don't think they had any PVC equipment on the valve covers, just a fill cap. Actual breathing wise, the crankcase will vent with the modern valve cover equipment and set up though.

 

If you're using modern heads, most of those issues go away. You can plug the road draft tube hole at the back of the block and use valve covers with PCV and oil fill provisions.

The dipstick will be on the driver side, which interferes with stock exhaust manifolds. Headers solves that problem.

The oil pan and front cover are different for those older engines vs. the '89.