What you need to do is
Measure from the centre to centre of the points where your shock mounts the frame. This is what is known as the i2i measurement
I'm guessing that yours will measure 190mm
Then. You need to know the stroke of the shock.
This is done by measuring the silver shaft piece that sits inside the spring( the bit that disappears when the shock is compressed)
I'm guessing that this is 50mm on yours
Them you need to know whether your bike is a single pivot design ala orange/ marins/Santa cruz or whether its a 4 bar linkage design ala specialized/intense/clubroost etc
With these bits of info and your weight, you can find out what weight spring you'd need for your Marin.
You also need to know which spring width you need. Older fox shocks like yours used a narrower diameter spring and shock. Whereas new shocks ie 04> used a bigger diameter. Make sure you get the right one or it won't fit your shock
Have a read of the tftuned website regarding this and the markings on certain brand springs as it will help you in the future.
Putting in these details myself (if they are correct and your frame having 4" rear travel then the outcome reads that you need a 489lb spring. So it makes sense to buy a 500lb one.
http://www.chainreactioncycles.com/Cate ... goryID=684
Try here. Nuke proof do both normal and oversized springs. Make sure that you go for non oversized(all manitou/rockshox/ti springs and instead choose nuke proof standard size. The only one I could see on the close to the size you need is a 500x2.25. Measure the distance from near the end of your threads on the shock to where the spring sits at the other end to see if it will fit though
Hope this helps