I'm working with the same conversion right now. The LS1 will work just fine in the 70 truck. From what I've read, so long as the tranny has a speed sensor in it, you'll be ok (your idle will work correctly).

Guys with the vette/z28 LS1 have had to notch their frames to fit the LS1 in. I "test" mounted my truck 5.3L in the frame last night and didn't see any problem aside from the power steering hoses hitting the frame. I'm going to be doing some more work with that this weekend so I can get my frame back to the powder coater. I haven't gotten my 4L60E from the tranny guy yet. I'm having him build a pre-97 4L60E that will have a pre-93? tail shaft and housing so I will have the proper mechanical spedo drive. He's also going to mount the reluctor ring for my speed sensor.

Just curious how you intend on handling the gear select linkage, throttle cable, and fuel delivery? I haven't decided how to do this quite yet, particularly if I mount the engine back to accomodate the frame clearance. I dont' want to cut the frame if at all possible. That's not an area that I want to weaken.