An impact at 45 MPH with an 180 lb person strapped in will put ~18,000 lb on the seat belt. The seat frame is mounted with 4 each 5/16" bolts thru 19 gauge sheet metal, so that's the weak point. Your angle iron just moves the failure point to the seat frame mounting bolts. The bolts may hold, but the sheet metal may rip out.
Keep your fingers crossed.
