Sorry, I did mean upsampling the seed time series. The TR is 1.5s and number of time points/volumes is 260; however, the stimuli onsets and durations occur outside of the TR. For example, the stimuli onset times are 5, 60, 115, 145, 200, 255, 285, and 340 seconds (total of 390 sec); and the stimulus durations are 25 seconds for the stimulus that occurs at 115 and 255 seconds and the duration is 50 seconds for the remainder of the stimuli. I hope that is more clear.

If I am understanding your advice about oversampling correctly, I think that upsampling by a factor of 3 (sub_TR of .5) and multiplying the stim times by 2 should do the trick? I tried this method and it appears to be working with matching 780-time points in the seed time series, stim file, and interaction files before downsampling back down to 260-time points again.