4500 is definatly the way to go, if your 6 speed is good you could sell it, buy a 4500, and have a few dollars left over for a real good clutch. There is a handy gear ratio calculator excel file in one of the pinned threads in the sled pulling forum. Generally, an NV4500 in 4th with a 241 in low range works well with a 4.88, though I pulled for years with 5.13's with no problem, and some guys try 4.56. Any one of those should put you in the happy zone!
If its a dedicated puller you can save some coin and buy an open carrier for the 4.10 and up gears, then weld the spider gears in it. Cheaper than a spool and just as strong. Stay away from the factory trac-lock or posi carriers, they will break in half!!
Then get on the track and break some sh!t!!! You'll find your weak spots quick enough!!! :rockwoot::rockwoot::rockwoot: