what size torque conv. on stock 66 c4

Sorry, the only listing in the 66 shop manual and specs. book is for the C6 which is listed as a 12". Your trans code should be PCW-AS. Maximum stock converter is 2000 rpm stall speed.

If you're not requiring a professional type TC. Both B&M and TCI make a decent torque converter for street & light strip use. BTW... I'd go for something like the TCI streetfighter 2800 stall...

I "think" you have a 24 spline, 10.5", 4-bolt pattern, as stock. Just be sure to order it for the '66 C-4 and it might be a good idea to count the splines on the input shaft, just be sure. Many early model trannys were replaced with 70+ models, when owners were getting rebuilds, back in the day.... which is a good thing, but they are different.