Definitely get an L37 unless you want a real headache. The computer and transmission are programmed for the L37 you have. You'd likely spend a lot of time and frustration trying to make it work right without faults and gremlins if you try the LD8 (unless you swap everything including the transaxle and the computer).
Regarding a replacement engine. I would recommend a remanufactured engine with a warranty. That's what I'd get if it was me in your spot.
Here's a link to a 3 yr / unlimited mile warrantied Northstar, that has been TimeSerted (very important procedure so that you don't get pulled headbolts and headgasket troubles).
http://cgi.ebay.com/ebaymotors/93-99...QQcmdZViewItem
Good luck. Let us know what you end up doing.