If it's a single row, I personally would change it at no more than 5-6° of stretch. My 4,5 had an outrageous 12° of stretch when I changed my chain out and I wouldn't let it go past 8 personally again. You will need to change your timing chain rails when you do the chain too, they should be cream-colored, yours probably look more like coffee (black). Often times the brittle rails break, leading to a jumped chain. My 4.5 has a dual-row chain, I didn't change the (aluminum-backed) rails or the cam gears, and had 1° of stretch (if that) after finishing. If I were you, I'd replace the chain and upper rails and then remeasure stretch, and if it's like 3° still, then replace the cam gear (assuming it hasn't been converted).

Conversion to double row chains are quite involved, easiest done with the engine out. If you don't pull the engine there's a lot of stuff to work around (radiator, condenser, etc) when doing the job. You HAVE to pull the timing cover and replace:

Cam gears (x2)
Crank gear
Distributor drive gear
Rails (lower and upper)
Tensioner rail (2 lower, 3 upper, IIRC)

This, of course, means removing the crank hub, which I'd imagine to be a pain in the ass with the engine in the car. The cost and complexity of this job is why the single-row chain is still available - many cars have not been converted.