L31 350 to (L21 or L29) 454

[VorTecxas]

I am swapping a '99 454 into my '96 Sierra 1500 w/ an L31.
(Stock for now, hot roddin' will begin when the ol' budget opens up)
I have a complete, running '99 Sierra 3500 4x2 (OBS) as the donor truck. I have a few questions I'm hoping ya'll can answer

1. How do I know if it is an L29 or L21? - Answered, L29

2. The donor truck has a 5spd, I'm going to be putting a 4L80E behind the 454 in my 1500. I know I'll have to shorten the driveshaft (going to put in a 1 pc shaft), move the crossmember, put a flexplate instead of flywheel, etc. But do I need to use a complete harness and PCM for an automatic? I have a complete harness with PCM for a Vortec 454 with a 4L80E (don't remember what year it came off of, I think it's '98, but I can recheck).

3. Do I have to use a '99 4L80E? If not, what other years will work with the '99 PCM? (if I can use it, 411 PCM in the near future, possibly be able to acquire before install)

4. I plan on putting my L31 back into the 3500 so I can resell it as a running truck, I'm pretty sure I'll have to change the pressure plate/clutch out for one for a 350, but do I need an adapter plate to bolt the 5spd trans to the block? What year/model should I get the plate and clutch for that will work with that trans?

5. Will my harness and PCM from my 1500 w/ auto work with the 5spd setup or do I need to find one for a manual?

I apologize for the lengthy questions, but I have searched around and can't find anything that answers any of them specifically. Thanks in advance for your input

 

[GMstrong]

I would love for the rpo code either L21 or L29,I think the l21 was only in the big commerical trucks though like teh 3500hd's with the booms and stuff like that,They had the coils right near the plugs like the 8.1L 496 LSx does.L21 i think was only mated to a allison and not the 4l80e trans so thats something to look for,i would 4l80e be hide it those things are monster trans!,

 

[454ss]

8.1 is not a ls engine,

 

[VorTecxas]

Noone knows anything about the PCM/ harness swapping? I'm trying to have everything lined up and ready to swap before I do anything

 

[VorTecxas]

Bump. Has noone had any experience with this?

 

[silver97]

I want to say you will have to use the 454/4l80e harness you have for question 2.
Question 3... Any 4l80e should work as long as the splines and transfer case will marry and has a place for all the connectors. I think as long as you got a 96 and up trans you will be ok.
Question 4. No adapter should be needed and you should be able to reuse your whole clutch system. I read on here that a guy used a clutch for a 454 on his 350 for more holding power.
question 5 I think you can use your pcm/harness from the 1500 but you will have to do something to the neutral safety to show it is in park.. I would research that a little more.

I am doing something similar I have a 97k2500 nv4500 4x4 and a 97k2500 suburban. I am putting the nv4500 in the suburban and the 4l80e in the pickup. So I will have to swap both pcms and harnesses.

 

[VorTecxas]

Thanks silver, I assumed as much as far as the harness, glad I won't have to get a new clutch and all that. That is if I end up doing it. Got an offer on the one ton today that would pay a few bills and still give me enough to find a complete setup ready to go, so I may go that route. Everyone keeps telling me to just LS it, which is tempting and I know what a 6.0 can do, all of our fleet trucks have them and for a bunch of HDs and duallys, they've got some cajones. But I just like the old BBC, and not alot of people do it.
Good luck with your swap, I was thinking of putting the 5 spd in my halfie, but after some thinking decided I'd rather just put an auto with a ratchet shifter, keeps the other hand free to explore the...uh....current seat cover...
Thanks again