SN74LVC8T245: About Input transition

The propagation delay is not the same as the output transition rate. Connecting an SN74LV240A output directly to an SN74LVC8T245 input will work.

Rout depends on the supply voltage, so you should use only the nearest V_OL/V_OH specification.

The outputs of TTL devices do not behave like CMOS outputs. As shown in figure 21 of the SN74LS240 datasheet, there are a 50 Ω resistor and a Darlington transistor (which should not affect the drive strength).

Please note that the output of a TLL device is not compatible with a 5 V CMOS input (V_OH is below V_IL(min)). The supply on the SN74LVC8T245's input side would have to be 3.3 V or less. (A 3.3 V signal is compatible with a TTL input.)

The LS output does not behave like a CMOS output, so you cannot use all these formulas.

You can assume that a LS output pulls up with about 50 Ω to about 3.5 V. This is more than enough for any CMOS input.