Transmission Compatibility

[gread]

Hello! I am excited to finally be posting on this forum. I recently inherited 1967 half ton from my father. I am planning on slowly fixing it up, to a practical level. It currently has a 250 inline 6 with a four speed transmission. First is a granny gear, I usually start in second. I don't need it pull extreme amounts, maybe a horse with a trailer at most. I saw a few posts on here before about a tremec transmission that many said was "virtually bolt on." But it was $2000. Are there any good compatibly transmissions that I can put in myself without jumping through hoops? I took auto in high school, but I don't know a lot about this sort of thing. Any advice is greatly appreciated.