LCT MK. 6

I'm not aware of any vehicle-type accessories in 1:400 scale.   That scale has been pretty much supplanted by 1:350 scale.    1:400 scale was [is] popular in Europe where Heller and Mirage are its greatest supporters.

If you cannot find anything in 1:400,   you may have to bite the bullet and go with some things which are 12.5% too small.    Iron Shipwright offers M4 Shermans, M7 Priests, some DUKWs and deuce-and-a-halfs in 1:350 scale. 

http://www.commanderseries.com/accessories.html

The alternative is 1:285 scale GHQ wargaming pieces.  They are 28% too big 

Hey Ed,

I disagree with your statement regarding 1/400 vs. 1/350. More plastic kits still available in 1/400 than 1/350. Only with resin kits is there a 1/350 advantage, but resin kits require much more labor in terms of parts cleanup of pieces on the part of the builder than PE upgrading 1/400 kits do. Plus the cost difference of kits. My favorite resin contremp is the lack of upper/lower hull alignment and bent or crudely shaped white metal pieces. Add in the inability to avoid voids in castings and the primitive pieces provided for masts and resin is still more of headache than plastic. There are a few foreign castings which are nice but I am referring to domestic USA manufacturers.

Only if you are a USN or IJN person is 1/350 better. Certainly not for German, Italian, French, Polish, and British Navy.

NB:  One fellow on modelwarships.com used 1/285 tanks in his 1/400 Heller landing craft. He shaved off the bottom of the kit. GHQ miniatures have sharp detail but 1/350 ISW stuff is closer in scale, although cruder.