Well, here's what I would do. given your situation and nudget.... (but others will have their own ideas)

Go ahead and swap in the SBC TPI and the TH350 auto tranny. Since you already have the Detroit Locker in the rear AMC 20, leave the rear axle as it is -- don't spend any more money on it. Wheel the thing as you want. If the AMC 20 holds up to the type of wheeling you do, fine. If it doesn't, then replace it with a stronger axle. Sell the Detroit Locker -- lots of guys out there with AMC20 rear axles who'd jump at the chance for a low cost used Detroit Locker. I wouldn't waste the time or money putting in a D44 for the rear axle. I'd jump right up to a Dana 60. That way, when you decide a few years from now to "add some more beans" to that SBC motor, you won't have to worry about whether your rear axle will take it.

Were it me, I'd look for a D60 axle for the front, too. Mainly so I'd have the same wheel bolt pattern front and rear. But it's not necessary. You can keep the D30 up front until/unless it breaks, and then put a D60 up there also.