With a 5:38 you would have a 35.5:1 (the 300 is 2.62:1) TH350 first gear is 2.52:1.
You should be fine with that. It may not be perfect but I wouldn't gear a D300. The 5.3 doesn't make near the power down low as he had but with the right converter and a heavy foot it shouldn't be a problem.
Mine in the buggy is:
30.99:1 in Mid range
55.79:1 in Low (which is too low IMO if you need any wheelspeed) It's nice for slow crawls when you just wanna sit back and chill.
I would really like something closer to that 40:1 range for my Mid range but with the new converter it pretty much does what it's supposed to.
Thanks Mason. That looks about like what I was looking for. Matt and I were talking about building the axles yesterday, but he couldnt remember what kind he had on his 60.