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.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.