Not sure which year your big block is, but if it has the L31 style water pump with the smaller thread on fan clutch it will work.

I had to do a little clearance of the fan shroud but it was relatively minor. The Duramax fan is about 1" larger diameter, but the blades are much wider and steeper pitched. With the Duramax fan my A/C blows ICE cubes even at idle on a 105*F day.

You need an early model Duramax fan part# 1580690

http://www.amazon.com/ACDelco-15-806...ywords=1580690

If your engine is a L29 with spin on fan you can likely use this clutch. I had Napa cross the Hayden 2851 number and bought their equivalent.

One word of warning this fan can get quite loud when the clutch kicks in (which is practically never) and you will notice atleast a 10 HP if not 15-20 HP loss when the clutch is engaged. That being said I noticed much less fan drag overall with the Duramax fan because it spins more time with the fan clutch unlocked than the stock fan setup.

If your tow pig has the old 4 bolt flange to it, might be able to use an early 6.5 turbo diesel clutch. The diesel guys are just bolting these fans onto their factory clutches.