1978 master cylinder predicament

I have a 1978 front mastercylinder with the rectangular resevoir and 1/2" piston made for single disk brake. Weird, I know, i just found out it was a 1978 only thing. I have located the rebuild kit at Redline cycles, but, impatient as i am i want to ride this weekend. can i use the 1/2" piston from a round resevoir with this mastercylinder? i can get them locally and could be riding tomorrow if it would work. Any thoughts?

i dont know man ive never tried mixing and matching brake parts.take parts from the 78 caliper with you to compare with.