Found a possibly useful resource--NMM Greenwich has a portion of their draughts collection scanned and posted online. Navigation is a little sticky, you basically have to search specifically for the ship you're looking for, but it's better than nothing.

For example, a couple 74's designed by Sir John Henslow where one supposedly evolved from the other...
1794 Mars:


1800 Courageux:


http://collections.rmg.co.uk/collect...eBy=collection
NOTE: Wide screens work better for viewing the images, as do 17" or bigger screens.

Of course, they'd like to SELL higher-res scans, but the "large" when you click the little double-headed arrow at top right of the image should be enough if you're just looking for "Can X stand in for Y?" level stuff. Sometime next week, I plan to toss up a thread with those images so we can try to figure out how far sculpts can be stretched...