TH400 long tail vs short tail - swap

I have a 76 Olds Cutlass I purchased a few months ago, with an Olds 350/TH400 combo. I have a 69 Buick 430 and TH400 combo to drop in its place. The trans in the car currently is a short tail. The 69 model is a long tail.
Anyone know if the input spline is the same? My thought is to shorten the driveshaft.
The trans in the car leaks. The trans from the Buick is solid-has 20K on a rebuild, shift kit, adjustable modulator.
This is a dirt cheap build for the Grassroots $2009 challenge. Every penny counts.  

The input shaft is the same between the 2 trans's, as far as the convertor is concerned.  The buick's shaft is usually different on the other end where it fits into the direct clutch hub, on most models.  
This is not a concern for rebuilders if you use the hub along with the shaft.
As far as the output length goes, GM made 3 different lengths of output shafts, depending on wheelbase is was used in, in an effort to keep the driveline as short as prectical.
These shafts are all the same diameter and spline size, so your Olds driveshaft yoke will fit.
Rather than shorten your driveshaft, my suggestion would be obtain the shorter output shaft and have your friendly tranny man change it out.  You said your trans leaked? Fix it all at the same time!  
The unit must be disassembled with all major components removed. Remove one snap-ring at the rear output reaction carrier and change the o/put shaft to whatever length you need, and re-assemble.  You'll need a complete gasket set and seals.  (about 50 bucks)
I've done many of these changes, and usually takes no more than an hour.  If rebuilding, naturally, it takes much longer.
Good luck, getting a new, shorter driveshaft made not only makes your drive-train a "bastard, if you will, but changes your pinion angle, which will need to be corrected or driveline vibration and excessive U-joint wear may occur.

Hope this helps....