I don't think I'd use the old style C4. The '70-'81 use a larger input shaft (among other things), and they will hold up better. Of course, you'll have to buy a new torque converter if you change trans. The flexplate tooth count depends on the size of the bellhousing (157t for the smallcase-fill, 164t for the larger pan-fill). See the C4 Forum for lots of good reading.
Is that a 351W? Did Ford change the balance on the 351W from 28 to 50 oz-in? The 302 balance changed to 50 oz-in around '80-'81.

<font size=-1>[ This Message was edited by: brianfulwood on 4/28/04 5:45am ]</font>