The following is based on a 15/64 drill bit test on 330 FT block:                       
4.63 bore spacing.  Subtract the drill bit diameter, 0.219, leaves 4.411 inches.                     
Allowing (subtracting) for two 1/8th inch (0.125”) cylinder walls leaves 4.161.                       
This is just over a 0.030 over bored 428 cylinder.  The 428 has a 4.13 bore; with a 0.030 over bore the bore is 4.16.                       
So if there isn’t much core shift I should be able to bore this block to use 428 pistons.                     

Or am I mssing something?

I agree with the others, drill bits might be a good idea to see if worth investigating further, but that's about it.  I don't even bother

You mentioned core shift, the only way to "see" it, is when you sonic check enough points to see show the entire pattern shifts on each bank, and each bank is generally different.  Usually, top bottom middle in every cylinder is enough to show the trend, but even then, it doesn't account for pitting, and certainly doesn't show the complete shape of a non-machined surface.

Nowadays, I try to do minimum bore and spend the money on a piston.  If that really is a thick block after sonic checking, I'd stop at 4.05 and build a snotty 390 or 437 at the biggest.

It is but its also a 68.

and a side-oiler

       A "sonic test" is nice, and yes, it does provide valuable information, but this can only provide an "impression", not an absolute value, of what one might get away with in the way of an over-bore capacity.  It's a "spot-check" operation, not an in-depth detailed study of the entire cylinder; and therefore there very well may be "anomalies" not realized in the testing effort. 

       I believe the best practice is to realize that the O.E.M. was not in the habit of proving excessive sums of material just so that someone down the road in a rebuilding process could 'punch' their "390" block out to the "428" bore size.  But, I might be inclined to believe that they did at times utilize the 428 water-jacket cores in some of the "390" bore intended blocks; the question to be answered would be "why"? 

       The first thoughts that come to mind might be, that this might be an acknowledgement from the O.E.M. that they had established that the cylinders "needed to be somewhat thicker" than as originally engineered; and this for an engine producing 'only' how much power?     

       Perhaps, the casting boxes were just getting tired, this resulting in excessive "core-shift" issues in the castings and an excessive sum of rejections; so someone said: we'll just substitute the "428" core which'll thicken-up the casting and allow it to absorb this problem.  Hey, that'd be cheaper than paying for all new casting boxes for an engine on it's last legs anyway!"?   

       Best possible scenario might be that the "390" water-jacket cores got damaged, so the "428" stuff was just substituted; but again, to offset the cost in the greater sums of casting material being utilized meant that there would be a cost savings in a reduction in rejections from cylinder wall thicknesses (somewhere) not meeting the minimum specification, even when the "core-shift" was great, the pouring was bad and the machining execution was sloppy!   

       So, your thinking "you" can go about making them thinner?     

       Scott.

       

It is but its also a 68.

I thought the cougar GT-E 427 was a hydraulic motor.