All but the bubblechucka are valid. Smasha gun is best all around tool,
KMK for some serious hurt vs T8 tough targets and traktor kannon vs -1/-2 to hit targets, especially flying vehicles.
HOWEVER they suffer from couple issues.
a) expensive as hell money wise. However since you like converting trukk box + some random bits(mainly for barrel) gives you 3 more chassis to use with mek gun bits so combine those 2 kits and you could get all 4. Or buy like 2 trukk's(6 chassis) and 3 mek guns(3 chassis + 9 good weapons) to give you 9 mek guns. 3 smasha gun, 3 traktor kannon, 3 kustom mega kannon
b) they are vehicles with low T, low save, 6 wounds in an army that often ends up swarm of infantry. No surprise where every single lascannon goes...Losing 3-4 in a turn isn't that unreasonable proposition if only vehicles you have are mek guns. Ergo you either bring LOTS of them(like dozen) or you bring other vehicles(battlewagons, deff dreads, gorkanauts), preferably with
KFF for 5++, to overload enemy
AT firepower. Infantry swarm with 3-4 mek guns will just give worthwhile targets for enemy
AT fire thus making your list infact worse off.