1961 Chevy C20 Apache with a 1967 drive train!

[61Chevy454]

Hey guys, I'm a Ford man but yesterday, I drove 3 hours and paid $3000 for a 1961 Chevy C20 Apache 4x4 long bed stepside with a 454 and a TH400. When I test drove the thing, it ran good, pretty much what you'd expect from an old truck. The motor has an Edelbrock performer intake, headers, a Mallory ignition box, and an Edelbrock carb.

About 37 miles from home, the thing wouldn't shift into second... It would go up to around 2000 RPM and then act like you put it in neutral. If you mash the thing, it will kick down into first and go. Sometimes, the RPM it goes into neutral at changes. After it does that, it will not down shift until you let it go really low and give it some gas. It has fluid in it. Occasionally, it will shift into second but not very often. It won't shift into third either, except once when I had it up to 60. I had to limp the thing 30 mph all the way back to Missoula... Took me 6 hours and I got home a 3:30AM last night. It did not magically heal itself over night, sadly, except when it was cold, it did go into second.

Any ideas?