Preventing Truncation of First Post

By truncating, do you mean using the_excerpt() rather than the_content()? Or do you mean using the_content() with <!--more--> tags ?.

In either case, you can alter how a post behaves by using the following:

<?php 
 if (have_posts()) : 
    while (have_posts()) : the_post(); 
        //We check that we are this is the first post in the loop AND
        //that we are on page one.
        //(Probably unnecessary) but to be safe you can uncomment the next line
        //global $wp_query;
        if(!is_paged() && $wp_query->current_post==0){
             //First post of first page, show full content using the_content();
             //This will display up to the first <!--more--> tag - not clear if you are using this.

        }else{
             //Otherwise display the post as normal / using the_excerpt();
        }

    endwhile; 
  endif; 
 ?>