LST project..

[Gustav]

Are you using the stock tranny.If so this makes placement of the batteries very difficult(read impossible)for good weight distribution.
A much better lay out is to use an e-maxx tranny or centre diff and to make an extended lower plate for the batts to sit either side of the chassis and lets you tune the front/rear balance.My suggestion is right back against the rear shocks.

If you want to use the stock tranny you'll have to lock it it 1st so you can remove the reverse and brake servo to make room for the motor,make a spur adapter for a smaller spur.you'll need 51 tooth and a large pinion gear 18T or 20T as the lst tranny is lower ratio than e-maxx 2nd gear.

The batts should go behind the tranny on the top deck(transverse).You can remove the plastic chassis brace to make more room(obvoiusly you'll need to keep some of it to hold the rear upper pin brace) if you brace between the bulhead/shock tower and chassis rails with tie rods and double up the rear lower plate.I use lots of additional bulkhead bracing on my LSTs any way.If you have an LST and drive it hard you'll already know why.