If you want to avoid any rules-debates and lengthy briefing sessions before the game, use them as a heavy bolter. A lot of players are very leary of homebrew rules (myself included), and would rather play a fun game than having to go over my opponents houserules with a fine-toothed comb. There are too many idiots out there that try and pawn off broken rules as brilliant creations. Case in point being some moron touting around rules for a 5000 point titan he brewed and that he supposedly uses all the time in small 40k games, or the guy making up his own marine chapter trying to weasel himself meltabombs at 3 points a pop.
There's also one other thing to consider: 40k has no set-in-stone formula to determine points cost. Determining the final cost is mostly setting it, playing a few games, tweaking it, playing a few more games, tweaking it some more, rinse, repeat. Simply using the cost of a vehicle upgrade won't work, as that price is based on the effectiveness of said item when mounted on a vehicle. Sometimes it's comparable, but more often than not, it won't work. E.g. a plasma gun costs 10 points in the hands of an IG veteran, but costs 8pts when handed to a Zealot. (Btw, zealots have hv. stubbers listed at 3pts per model).
Just use your heavy stubbers as heavy bolters, an item for which there are already rules in place. A HS easily falls within the confines of WYSIWYG, so I doubt anyone would give you any problems if you were to attend a GT. As a matter of fact, the Genswick Rifles IG conversion used Ork dreadnought weapons to represent heavy bolters (or maybe autocannons... don't remember which one), and that conversion was on the GW website, taking part in a GT, and got its own WD article.