The earliest 351W block is supposed to be a hair shorter I think. The normal spec is 9.50" block deck height, note the Cleveland block is 9.2", shorter by .30" which makes a bigger difference(intakes etc).

That could only affect the compression very slightly, and the fitment of an intake manifold. In any case you can still mix the parts if you take normal assembly steps, checking deck height and test fitting the intake before tightening bolts.

It's like the first 289 blocks that had just five bell housing bolts, true but they still are usable.

nominal deck heights are 9.48" and 9.5", makes no difference unless you're doing internal engine work like replacing pistons or doing a stroker. my 1968 302 had a 3 bolt harmonic balancer vs the common later (1969+?)4 bolt hub. i'm not positive but i suspect the balancers are interchangeable? IIRC the 68's also used a unique (rare and expensive / valuable) cast alloy front accessory mounting bracket but i forget ATM if it was for the alternator od PS pump. my 68 302 also had a right side lower radiator hose on the water pump, which again should be able to be updated to the left side hose water pump. so the jist of it all is that the 1968 front accessory package including the balancer and water pump will need to be updated to the later style package