About the length of the arm on the primestar 40 x 30 oval dish, it actually depends on the feedhorn used. There were 2 different feedhorns used. One is about 2.6 inches in diamaeter. The other was larger (about 3 inches I think). Anyway, the smaller diameter feedhorn used a very slightly longer arm whereas the larger diameter feed used a very slightly shorter arm. This was done to place the feeds at just the optimum focal point for each one. look for a sticker on the bottom of your arm. If it was used with 2.6 diameter feedhorn it will say something to the effect that the arm should only be used for 84E dishes (this is the 40 x 30 model, the 36 x 26 is 74E) and that it should only be used with the 2.6 feedhorn.
Anyway, as for LNB on the primestar dishes, if you look at the stock LNB and feedhorn you'll notice they used a separate feedhorn and LNB. That is the feedhorn and LNB are 2 separate peices bolted together. This is what differentiates an LNB from an LNBF. An LNBF is an LNB with the feedhorn made onto it. That's what the "F" on the end stands for. Anyway, the primestars have a feedhorn with what is known as a "C120 Flange". In fact, I believe most all of the Channel master primestar and dircet PC dishes used these. What I recommend doing is sticking with the original Primestar feedhorn because it is optimised for the size, shape, F/D ratio, and focal point of the dish. An LNBF will not be. An LNBF will be optimised for whatever dish the manufacturer of it recommends to use it with. Therefore, I recommend either using the stock primestar LNB with the feedhorn or purchasing an LNB that mounts to a C120 Flange and attaching it to the primestar feedhorn. This is the only way you'll likely get optimum performance. I personally use a stock primestar LNB with my 84E and it seems to perform quite well. However, if you must use an aftermarket LNB, Invacom makes a C120 flange LNB. To be quite honest though, the original primestar LNBs might not actually be a bad choice. One thing to remember about LNBs is there are many parameters that affect their performance and are quite important other than just simply the noise figure.