The only alternate lower for the upper 5.0 H.O. manifold is the Edelbrock. The problem with the Edelbrock manifold is you need to hog out the ports on the heads to use it. TMoss sells a very good quality ported factory lower for $125 or so. If there are other lowers I haven't seen or heard about them.
E-brock does not make a lower that fits the stock upper that I'm aware of.
Holley Systemax 1 lower is made specifically for the stock upper, if you can find one... I've been told they stop'd making them? I hear Total Engine Airflow and Ed Curtis have some laying around, but be prepared to drop 250+ for one.
I bought one of the last ones Summit had, the port alignment was terrible and the runners were huge... I spent a bunch of coin having it ported to work with stock heads, and, to make a long story short...
For 100bux plus shipping it would be hard to beat TMoss's stock welded/ported lower, this is the path I am now on.
Nifty.
Summitt was selling them for 270 some odd dollars.
With the very very crappy port alignment you would be far better off going with one of Tmoss's units, especially considering Tom's lowers will out flow the stock upper.
If you plan on running a box upper, or spending some huge dollars modifying the stock upper to get the flow up, the Holley would definatly be the route to go. My modified Holley lower flows 276cfm(with more room), but, with my heads flowing 205cfm and a stock upper stuck in the mid 190cfm's the Holley just does not make sence given the dollars invested in it...