+1 for Chucksbikes
Built up this 61 cm frame last winter into a cheap ~18-19 lb bike:
I'd ask if it would be ok for a 280 lb guy. Supposed to be the same frame as used in the 2004 Fuji Roubaix. The 61cm is measured to the top of the seat tube. The top tube is 59cm, which turned out to be a bit long for me (using 90mm stem now). Chucksbikes also supplied good inexpensive two bolt seatpost, handlebars, and stem. It has a 130mm rear dropout so your 126mm rear wheel will need some extra washers, and possibly a longer axle, but the frame would be ready for an 8-9-10 speed wheel someday.
Or you should be able to find a quality Japanese steel frame & fork from the 1980's for around $100 more or less, used, online. My good steel frame bikes built up to about 22 pounds, and cost less than $50 at yard sales and thrift shops. If you see a used bike with Shimano Dura Ace, Ultegra, 600, 105, or Campy hardware the frame is likely to be a good one.
Avoid most cheap Ross bikes! Old low-end steel bikes have thick heavy straight gauge steel tubing. Quality steel frames used stronger steel (usually Cro-Mo), which can be made thinner and lighter. The good tubing is butted (tapered, double triple or quad butted, sometimes "channeled"). Newer steel, like Reynolds 853, "air hardens" and makes strong joints without the lugs and brazed joints used on the better older steel frames. Cheap bikes may have mild steel instead of forged steel dropouts.