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.

It could have either 3 or 4.  The shift rails are not the same.  What do the dog teeth on 2nd and 3rd look like?  If they are worn, then the ends of the splines on the slider are most likely worn also and that greatly affects downshifting. Even if the dog teeth, on the gears, were perfect, if the ends of the splines, on the slider are worn it won't shift smoothly.

I don't like the needle bearing pilot bearings.  If that has failed 2-3 and 3-2 shifting will be unsatisfactory. I only use "bronze" pilot bushings.

I use GL5 in my 69 Mustang vintage road race car, it shifts rather good and the blocker rings are not showing abnormal wear.  What I have read about the current GL5 gear oils is that they have to be at boiling temperature to damage brass blocker rings.

My block is dated Jan 16 1970 and doesn't have the ribs. 

The drill bit barely goes in near the center and won't move up and down more than an inch. 

It has the reinforced mains.