Hi Dennis,
Welcome to the TI E2E forums!
We don't have an IBIS model for the ADS1251 to date. The ADS1251 was released in 2001 and IBIS models were not often requested.
The ADS1255 is a different design and comes in different packing, so I would not expect it to be the best match to the ADS1251. It might suffice as a rough estimate though.
Regards,
Chris