motor sounds like its ready to let go on ya .how about putting a sleeve in scored block and go next oversize piston's, seems like the 2 motors will not interchange , maybe try to sell the 43 motor and use that money to rebuild the 53 motor, 1 sleeve is only about $ 30.00 or just re bore to next over size if it will take it, regrind crank if it needs it otherwise clean it up with #1,000 grit, lap it in and go with it, you can always go with the turbo pistons and ad turbo system,a major rebuild kit cant be that much maybe ballpark figure of $1,800 after market or $3,000 oem cat, say $ 500.00 in machine work maybe ,fest