I'm told their is a better selection of tq converters for the 27-spline... is it possibel to convert the input shaft or am I better on getting a newew 904 for my build?
Your welcome. Yes there is a much better selection for 27 spline units. I have worked on a bunch of 727's and it's easy to convert them over to the newer spline but I've never done a 904 conversion. I'd assume since they're basically the same trans it'd be the same conversion. I can't say positively but I bet if you'd get a rear clutch assy. (that's the one the input shaft is attached to) from a 68 or 69 unit it'd swap right in. They made some other changes in 70 so I'm not sure if they'd work.
Also don't forget your 66 engine will have the small pilot hole in the crank. That could be a problem because unless it's custom made converter they don't make a 27 spline converter with a small hub.