Hi Tom,
You got lucky with the TH400 in the truck. Even though the low gear is a shade higher ratio than the TH350, the 400 will last a lot longer and give you better service. Remember what I said, though, about making sure you get a 292 block that already has 3 starter bolt holes so you can use whatever flexplate diameter you want.
I didn't say anything about my '63, but I'm in the (long) process of updating the original drivetrain (292 w/4 speed granny) to a monster turbo 292 with a TH400. The turbo motor is a while off, so I've had to bide my time with installation of the periferals. I found out the hard way that you can't find a starter with the old-style parallel bolt holes for a large diameter flexplate. All the parts stores have only the offset hole nosecones. And I had already committed to using a large diameter flexplate and heavy-duty large torque converter. So I had to drill the '63 block for the missing offset hole. The turbo motor block is a later model, so already has all 3 holes.
As far as the kickdown switch goes, some of the '67s through (some) '69s had a switch that was mounted at the back of the carb for the TH400 applications. I've still got one of those old things, though I probably wouldn't use it on anything I've got. The newer models all used a switch that mounts on the gas pedal inside the cab. I've converted most of my stuff on the '63 over to late '70's Buick parts, so that's the kind of footfeed and kickdown switch that I prefer.
Now on to the dual exhaust! Yeah, the engine you are visualising was an "integral head" 250 that used a progressive 2-bbl carb. Integral meaning the intake and exhaust manifolds were part of the head casting. They were fundamentally garbage engines, although one could swap everything above the block deck to the older style 194/230/250/292 top end if the head and/or manifolds cracked. They had a pretty bad reputation for that.
Good luck on the improvements to your peekup, and give a holler with any more questions.
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David
