yep..your asking for pain cramming shocks on. To find out what shocks will fit, lift the bike in the air , take the shocks off, measure from the upper shock mount to the lower, thats the extended length. then set you bike down with out shocks, lift the frame about 2 inches off the ground and measure from the upper to lower shock mounts again. thats the compressed length you need. now that you have the exended and collapsed length, subtract them..
15" extended - 12.5" compressed length = 2.5" of shaft travel/length (just usung random numbers) then subtract 0.125 ~0.250" from the extended length (safety margin) so now that you know this, say your looking at shocks on ebay off who knows what, send the seller a message to measure the length of the shock eye to eye and the length of the shaft and you will know if they fit.
Realize too, that creativity can be your friend... this one time, at band camp, when I first started messing with blaster suspenion, I made some plates that bolted to the upper shock mounts, spending a day cycling the a-arms and measuring different points I managed to get a set of works tripple rates fro a 250r to fit by changing the location of the upper mount and without anything binding. it was a long time ago, but I know it added a couple inches of wheel travel on stock arms.
there is a lot of aftermarket overpriced junk out there, if you use a little creativty, engineering and geometry, you can usually make better for less. if your in highschool metalshop and can mig weld... no reason your bike cant have 12" of travel on a +2 swinger for less than $50.oo