I've not scanned negatives before for large format printing, but, alas, I want to make some prints larger than my darkroom/wallet will allow, so I need to scan a couple negatives, but here is my problem.

I want to make a couple 30x40s from black and white 4x5 negatives, and I have access to an epson v700 scanner.
All scanner quality issues aside, how should I scan my negatives? I know the scanner has a maximum optical resolution of 6400dpi. Should I scan my negs at an output 4x5 inches and 6400dpi and resize in photoshop, or should I scan them at like 300dpi with an output size of 30x40 inches? My printer (El-Co) asks that files be sized accordingly to the size I want printed at no more than 254dpi, so should I scan at 254dpi and 30x40in?

my negatives are portraits with tmax100 and txp320

thanks