[Karter2]

Any production EV pack (or even reconfigured cells), will blow your weight limit .
Ampaholic or good 100C, RC lipo is about your only option unless you have access to non commercial SAFT or A123's F1 pack cells.
As Brian said , using multiple parallel RC packs (5x4Ah ?) and possibly reterminating the cells to buss bars, is your closest DIY chance using commercial products.
But even a minimal capacity pack (8-10 KWh) will still be 100kg of cells

 

[Karter2]

The ampahaulic/John Metric cells are on the list and in the running. Looks like we would need a 110s4p pack of those cells to meet the needs. Cell weight only would be 127 lbs but as stated the $12k price but that is the almost the price of a small block long block less blower/turbos.

Looks like the Volt Gen 1 or Gen 2 battery packs may be able to fit the bill if they can be pushed to 16 C for the 90 seconds. Chevy rates them at 7.8 C. Reports online show they can be pushed to 20 C but length of load test is kind of unknown. ........

It sounds like you certainly know what you are doing , but to be clear.
Ampaholic cells are expensive because you are buying the best , high discharge , (200C) chemistry.... Which you wont be needing...but they will certainly do the job.
The cheaper (??) Volt pack may work for you if it can stand the higher output ( needs to be tested ) and would certainly require cooling systems. However that all comes at a weight penalty (200+kg) and a size/ shape factor to work with.
There are other high "C" rate, RC lipo options that could also work , and cost less than either of the above and weigh in at around 50kg, (and could easily be split into 2 separate 400v packs)
It depends what your prime critera are, Performance, Cost, Weight, size, life cycles, reliability, availability, etc etc
Most EV builders would not consider using RC lipo due to its low life cycle, and high maintenance /monitoring requirements(for safety). But for competition/ race use, it is difficult to ignor its advantages.
If you have the budget , go with Ampaholic, if not, consider other RC lopo options.