Lew are you near Evansville, IN? Contact Champ Converters, they may be able to change the splines for you in short notice. When I get to the shop, I will add contact information to this post.
Has the input shaft been changed. I'm not sure about the splines just yet, I thought the C4 shafts were 24 and 26 splines.
I will get back with more information in a bit.
The C6 uses a 31 spline shaft. Can you replace the big shaft with the small shaft? What is the spline at the pump end?
Call Champ Converters in Evansville,IN. Talk with Kurt. 812-424-2602
Kurt (Champ Converters) built the converter I use in my '68 Mustang with FE/C4 combination. Kurt went out of his way to correct a converter/transmission depth issue, and corrected it while I waited! Like previously mentioned, I too built a C6 converter for the FE to help with re-sale. I use a spline adapter from Jim Paquet (JPT, Eaton rapids, MI) to use the C4 shaft with the C6 converter spline. It all works great.....so far!
Good luck, and don't give up yet, you have a week!
Masters Degree in Perverted Idiocy.