Aerostar 1993 4.0L. Timing Chain Help!

Ok, I just replaced the heads on my 1993 Aerostar 4L. Ran great. Was doing the "test drive" last night and was able to put about 10 miles on it. Cam
up to a stop light and it went "Kunk" and died. Turns over fine. Spark is there. Gas is there. No compression is there. :( . Figure the new heads/springs did in the timing chain at 250K miles. My bad for not replacing it while I was already in there.
Is it possible to replace the chain with the engine in the truck?? The book says to drop the oil pan?!? I have the front wheel drive crap and the frame there. Is it possible to do without pulling the pan? Ideas please?
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