tranny problems

hey everyone new here...im having a tranny problem and dont know what it is.....i just recently put a 355 small block in my car pushing around 400 horse and i still have the stock 700R4 tranny in....i had hooked up the tv cable and everything and when i reach about 45 mph just before it should shift to third gear it slips as if it went into nuetral and i dont understand cuz the tranny only has 32000 original miles and being that its a new engine i dont floor it because i need to break the engine in....please help