those of you with 55-59's

[a58chevy]

Is anyone still running the stock steering and straight axle with a V8? Mine has the original 283 (warmed up a bit) and I'm having trouble finding any headers that fit the stock set up! Also I want to put a TH350 in it. What needs to be done with these trucks? It's a manual right now.
Do I need new motor mounts with the auto, new crossmember? I know the driveshaft might have to be re-sized. Also what about shifters? I don't have any experience with aftermarket auto shifters. Would i need to fab up something to make a strip style shifter mount in easy reach?

Looking to eventually make this a pretty serious street/strip truck with a gasser style (hence keeping the straight axle).