First, as far as aftermarket, plastic .50 cal MG's go, Tasca's offerings are hard to beat. While the Academy set offers a lots of options and parts, the MG's themselves have a few issues, mainly with regards to the diameter of the cooling sleeve and of the muzzle section. As Frank mentioned, the Tasca M2's are well done and don't even need replacement barrels. As a matter of fact, they compare quite favorably with even the best resin/multimedia options.
As for vehicle mounts, you should know that the mounts for Jeeps and Beeps were not the same. Jeeps used the M31 or M31C mount that was mounted directly onto the floor of the vehicle, whereas Beeps used the M24A1 or M24A2 mount that actually replaced the forward wall of the cargo bed and was screwed to the floor and sides of the latter. Although I don't have a scan of the M24 mounts, I did include a scan of the complete M31C mount in my article about .50 mounts which can be found on Tim Streeter's "Modeling the US Army in WWII" website (www.usarmymodels.com). Here's a direct link to the page that shows the M31C mount:
The AFV Club (ex-Skybow) MC-52 "Beep" kit includes a M24A2 mount as an option. As for M31 mounts, Skybow did offer the M31C mount included in its original M38A1 Jeep kit as a separate item, but it lacks the three stabalizing braces. I don't think AFV Club offers the set, so these might be hard to come by. If you're really desperate for a Skybow M31C mount, drop me an e-mail as I may have an extra one in my stash.
Hope this helps.
Hervé "Charby" Charbonneau
You should never underestimate the predictability of stupidity.
(Bullet Tooth Tony)