Speed 400 with 24oz plane
Hi BSDGuy - Yes, whatever you do with a 400 is going to be less than perfect, but if you can keep the weight down to the 24oz (and thus the wingloading under 7oz/sq.ft) then you have the ideal characteristics for a trainer. Going heavier and faster defeats the purpose! The suggestion of loading your motor to 10-12amps would, in my opinion, be a recipe for disaster as your motor would fry in short order - in my experience "basic can 400's" tend to overheat quickly and run erratically at over 8amps or so.
I don't have figures for a 6V 400 - mine are all 7.2V but here are some figures for them anyway. If you use a 3:1 gearbox, an 11x5.5 or 10x7 prop and 8-cells (600AE NiCd or 950KAN NiMH) you can get peak static thrust figures of almost 15oz at 7-8amps (a more reasonable and sustainable figure is probably about 12oz at 6amps). That would give you the "rule of thumb" figure of 1/2 the weight of the plane and it should fly okay.
If you want to spend some money now but have a long-lived and maintenance-free future, the simple solution is to go brushless. Here there are lots of options. (For comparison: a Speed 400 with MPI ACC 347 3:1 g/box, adaptor and 10x7 Aeronaut prop weighs, in total, about 130g).
The cheapest route would be to use an MP Jet 25/35-20 [it comes geared 5:1 and you could use a 10"-11" prop: total weight = 118g] which would save some weight, yet give you a sustainable 14-16oz @ 8amps (and a momentary peak of 22oz at 13amps!).
A Mini AC1215/20 (at 108g, again lighter than the 400) with a 2.33:1 or 3:1 will give you 14-16oz @ 10amps with a 9x6 and, with a 10x7, momentarily as much as 20oz @ 13amps. At around 8amps you'll still get 13oz or so.
An AXI 2808/20 (110g) saves you the weight of a gearbox and with a 10x7 can give you 13-14oz at (for it) an easily sustainable 8amps. If you go crazy with it it can deliver, momentarily, more than 20oz at 16-20amps!.
Hope this helps, Cheers, Phil