Welcome to the fam. It won't be that noticable when you switch to the new XL TVPs. It will definately handle better and be more stable, but some people say that the turn radius is increased when you run with an ext. chassis. If you switch to bigger rims/tires, that's when you'll notice a difference in speed and accel. If this helps, I made my own TVP's about a year ago and I made them, they finished out to be longer than the XL TVPs. I'm also runnin the 25 motor until I get the money for a bigger one. As far as your question about the easy access trans case and diffs, unless you are gonna upgrade to the 3-speed (if you don't already have one) then the trans case isn't all that important. But, if you are runnin or gonna run the 3-speed, then upgrade to the X-style trans case. It'll make it alot easier if you have to make shift point adjustments to the 1-2 shift point. The 1-2 shift point isn't accessable from outside the case. You have to pull the center gear set out and remove the 3rd gear and 3rd gear clutch and slide the 2nd gear out a little to access the 2nd gear clutch. Now for the diff cases. Since you are already in the mood for upgrade, I definately recommend upgrading to the X-style diff bulkheads. It makes diff rebuilds alot easier and less time. Also, when it comes time to shim the diffs, it makes that alot easier too. I know this is quite a bit to read, but it's some good info to help you get started. And again, welcome to the HPISF family.
P.S. If I missed anything, somebody will fill it in.
