478 V6 Into '72 K20

[1972RedNeck]

Found a '73 478M for my '72 Chevy 4X4. Have the cross member, motor mounts, accessories, and the like pretty much figured out. I'm still not decided on the transmission though. The axles will be a Dana 70 rear (slid 6" back with 63" spring swap) and Dana 60 front with 3.54 gears. I want to add a .77:1 to .73:1 overdrive on top of that (80 MPH speed limit here in MT - 90 MPH isn't much of a stretch most times).

I have come up with 2 options that I like.

The first is to get a custom flywheel and run the stock SM465 married to the stock NP205 all bolted to a stock V6 pickup bell housing. Then add a 3 speed auxiliary tranny behind the transfer case. This would give me overdrive and great gear spacing for towing.

Option two is to keep the SAE adapter and flywheel (will have the whole rotating assembly balanced) and run a 7 speed Spicer and divorced NP205 transfer case.

This would be a little cleaner with good gear spacing and a nice .73:1 overdrive.

The thing is, I'm not sure that the SAE adapter will fit in the truck without heavy firewall mods (which I won't do).

Can anyone tell me for sure if the SAE setup will or will not fit?

Does anyone see any holes in my plans?

[bigblockv6]

I've seen where the SAE bellhousing used on the 60-66 style trucks and there seemed to be enough space for it. The 67-72 style trucks are a lower profile design so most likely there wouldn't be enough space unless you did some modifying or a body lift. I stuck with the pickup style bellhousing and SM 465 trans on my 478 conversion and quite happy how it came out because I retained the stock appearance.

[1972RedNeck]

Originally Posted by bigblockv6

I've seen where the SAE bellhousing used on the 60-66 style trucks and there seemed to be enough space for it. The 67-72 style trucks are a lower profile design so most likely there wouldn't be enough space unless you did some modifying or a body lift. I stuck with the pickup style bellhousing and SM 465 trans on my 478 conversion and quite happy how it came out because I retained the stock appearance.

What did you do for rear end gears?

I would like to be around 2K or even less at 80 MPH. 3.54's, overdrive, and 34" tires works out to be 2100 ish RPM at 80 - not ideal, but 3.07's may be too fast. 1800 RPM at 80 and 1600 RPM at 70 - still not sure how it would be even with 530+ cubes... Might be alright - would probably be able to get some decent mileage numbers...

SM465 and a 3 speed Brownie would be cool - 24 possible gear ratios. I'm not sure I have room to get 4 shifter levers through the floor though...

[bigblockv6]

Right now I have the stock 4.56 gears with 33 inch tires as it was from the factory. With that many cubic inches and torque I don't need that kind of gearing so either 4.11's or 3.73 would be about right for my needs and possibly an overdrive.

[1972RedNeck]

Originally Posted by bigblockv6

Right now I have the stock 4.56 gears with 33 inch tires as it was from the factory. With that many cubic inches and torque I don't need that kind of gearing so either 4.11's or 3.73 would be about right for my needs and possibly an overdrive.

How fast do you normally cruise? What RPM's feel like the "sweet spot" for your 478? Yours has stock compression ratio and a reground cam, correct?