i would go with #2, cuz i luv the fastbacks. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/inlove.gif" border="0" valign="absmiddle" alt='

'> plus, u'd have a fair amount of money to use on upgrades.
#3 seems like a better option because u'd have a lighter car than the FB. plus, iirc, the silvia front end is about 20 lbs or so lighter than the 240SX front end.
also, i don't think u should worry about bein in a higher class if u swap in an SR. that should only matter if u intend on seriously competing in AutoX. if anything, u should want to be in the higher class, just so that u can run earlier than the Stock Touring class.