I don't think it's a good idea to do it in Hires mode. Such a game needs to have at least 25 ips (Images per second), and with large graphics in Hires this could be to difficult to achieve. Chip Ram is really slow, but if the Hires mode is activated, it's even slower, and this not only because of the screen taking 2x as much memory as a LORES screen, but also because in that mode processor and blitter gets less cycles to work with each second. If you want the sprites as big as in the real Arcade game, and well working on an original Amiga 500, then you better do it in 320X256 (LORES) with 32 or 64 HB mode.
But this is of course just my opinion, and you would really surprise me if you manage to do it in HIRES
Edit: Maybe you could also use real (hardware) sprites to help you get more colors. I mean, a mix of bitmaps with sprites. If i'm not wrong, sprites can be as high as 256 pixels !