i have 39.5s, had a mini front axle for a while with them. no hub studs, no cromo hub gears, stock out puts, 5.29s, reg 4 stud arms, 3" spacers and drove it very hard. only thing i every broke was the steering arm pin. i've always had an fj80 rear axle so im not sure how long the 8" R&P would hold up. i now have an 80 front axle wich still has an 8" 3rd that been holding up fine. you will def need 4.7s, i use lo lo all the time. my next upgrade is outputs, then im golden.
on another note austin(with the yellow first gen) does 90% of the stuff i do with little struggle on 37" mtrs. my opinion is really dont need 40s on a wheel base like yours