> So, I would just write 60*15 90*25 120*15, etc..?

Yes, that's right.

> Would I combine the two conditions into one stim file?

No, you need to create two separate files.

> or can they still be separated to use -gltsym later, e.g., +easy -difficult

For each condition, you would get two regression coefficients for each condition, one for the condition effect that is associated with the average value of the modulation variable (e.g., average distractors), and the other for the marginal effect (slope) of the modulation variable (e.g., percent signal change when the number of distractors increases by 1). So when you construct a general linear test, you need to specify which coefficient to compare. For example,

+easy[0] -difficult[0]
+easy[1] -difficult[1]

Gang