My son did drive the car 600 miles without incident, it did just fine once it got out of first gear (only had to run through the gears three times). Currently up on stands, we've got the old one out and with the transmission in situ, it looks like the new one should go in without much of a clearance problem.
One question, though: Concerning the new bushing in which the 'nose' of the new piston rests, should it be RTV'd or otherwise secured in place - it doesn't seem tight enough for a friction fit to be safe? The old bushing is soft rubber (more of a seal than a bushing) and is glued in place; the new one is rigid nylon and more of a guide. Any special sealants recommended for use inside a transmission?
A pic of new and old is at
http://savage.mbz.org/IMG_1125.jpg
Thanks.