I have a Barry Grant fuel pump and I need to put a filter in. Is there a certain distance I should mantain between filter and pump? Or Can I use a coupling and have them connected? Can the nut and sleeve for going from a hard line to a fitting be reused? How big is a 10AN fitting?

It's best to have prefilter and a post filter. The prefilter should be large and free flowing, about 100 micron for a BG rotary vane pump. The prefilter just catches the big stuff that might effect the pump. The factory used a "sock" filter on the pickup for this task. The prefilter can be mounted anywhere between the tank/cell and the pump including attached to the tank or pump.

A post filter (about 40 micron for a carb) should also be used after the pump. It can be installed anywhere between the pump and the carb (some prefer it before the regulator, some after, there are arguments for both). The post filter uses a lower micron rating to catch the smaller stuff that might effect the carb. The factory installed the post filter in the line or in the inlet of the carb.

More info here:

http://www.robbmcperformance.com/products/filters.html

Mopar Performance books will advise you to "NEVER USE A GLASS OR PLASTIC FILTER. ALWAYS USE A SEALED METAL FILTER IN THE EVENT OF A CRASH, YOU DON"T WANT THE FILTER TO BREAK OR MELT AND CAUSE YOU TO BURN TO DEATH" or words to that effect. Which is probably pretty good advice.