I think the Comet “retro bullet” style is the best option on the market for EM machines; the color and brightness is as close to incandescent as it gets, and the slightly taller bottle has identical dimensions as the glass bulbs, so the appearance is maintained.
Like the Comet guy said, all wedge/bayonet GI/insert bulbs in virtually all pins run at 6v, with the exception of JJP. I’d personally be careful with Spike system (I’ve never modded the lighting on one); while any 6v LED will “work”, I believe the system uses current sensing as part of the protection circuit, so if you swapped out the OEM 2-smd bulbs for ones that pull more current, I suppose it could cause some glitches.
Also, Comet’s QC is fantastic; my rate of “dud” bulbs and ones that quickly lose brightness is practically nil with them, which is especially appreciated for those spots that are buried 6 layers deep in plastics and ramps and obscured by a drop assembly below!