Watch Craigslist or other listing services, and try and buy a complete running car. This way you know better what you have, and there are a lot of parts that you can use on the conversion ( trans, drive shafts, wiring harness, fuse box, Exhaust downpipe, ECM, extra wire, etc...) . I've found two donors for less than $400 each. One the owner couldn't get to run properly ( spark plug wires were crossed when he changed plugs, bad fuel pump was the real problem). They are not as prevelant now since CL started charging for auto ads, but try placing a wanted ad on CL / facebook.
'87 GT , '00 3800 Series II SC, 4t65e, Vue Power Steering. (SOLD)