All the GNE native ECW Olympians were of same length and height. However some were different internally with DP seatings. Also there was differences between the batches mechanically.

3572 - 3611 (JTY372X - JTY407X, JTY368X - JTY371X) - Were powered by Gardner 6LXB engines, basically standard Olympian at the time.
3612 - 3617 (SJR612Y - SJR617Y) - Were also powered by Gardner 6LXB but had a different gearbox, I believe fully automatic rather than semi on the X regs.
3520 - 3523 (C520LJR - C523LJR) - Cummins L10 with Voith 4 speed gearbox.
3649 - 3673 (C649LJR - C673LJR) - Cummins L10 with Voith 4 speed gearbox.
3674 (C674LJR) - Gardner 5LXCT with Voith 4 speed gearbox, experimental vehicle and only one of a handful of Olympians fitted with this combination.
3734 - 3748 (B734GCN - B748GCN) - Cummins L10 with Voith 4 speed gearbox. Were also the first batch of Olympians to be fitted with Cummins L10 engines.