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Discussion Starter · #1 ·
I have a small block and a TH400 that I would like to install in my '73.
My problem is that I can't figure out a transmission mount.
The thing is, for those who don't know, '73s never came with TH400s. '68 to '72s had them, but only behind big blocks, and the big block engine and tranny mounts were offset to clear the steering.
The way I figure it, my options are these:

1. Skip the TH400 and use a TH350.
As far as I know, the crossmember and driveshaft that I already have (three speed manual) will work for a TH350, and then I can pick up a cooler, shifter, pedals, etc, and sell the TH400 to pay some of it back.

2. Buy a custom crossmember.
Summit has a TH400 crossmember that looks like it would work, but it's around $230, and money is tight. If I'm not mistaken, I would also have to do some work with the driveshaft as well.

3. Switch engine and transmission mounts to offset big block mounts.
I'm curious if I could get away with '68 to '72 big block mounts with a small block. Buying OEM big block engine and tranny mounts would only run me about $120 as compared to $230. I would like to have a big block in there at some point in the future as well.

I don't have the fabrication skills or equipment to make a crossmember, and I want to make sure it can pass tech at a drag strip as it is my race project.
 

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I ran into the same exact problem as you about 3 weeks ago, I had to order one off ebay, I called the company to confirm that it will work with a SB and TH400 trans and they said 100% it will, the guy was very helpful with other questions I had and seemed to know what he was talking about, this crossmember has the plate in the center and is not offset, you WILL have to drill one hole on either side of the frame to get it to bolt down with 2 bolts on either side, one of the holes arlready in the frame you will use, no big deal, I paid $130 for it, go to Ebay and type in "TH400 crossmember SB" and it will come up. Worked perfect for me other than the fact that one of my rear pan bolts wont be accessible as the mount is in the way. Yes you will need a different driveshaft, mine was shortened. You will read that the TH400 is not needed unless you have a high HP set up and that it will take a bit more HP to turn, its heaiver, etc, the way I see it is im set up for future mods (and it was already in the car when I bought it)
 

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Discussion Starter · #3 ·
Yeah, it seems silly to be running a TH400 behind what is basically a stock small block, but in the long run I'll be glad I have it when I want to put more power down. The reputation the TH400 has for excellent reliability and design is rather appealing as well.
I've come across a few of those ebay crossmembers, listed for a 1967-69 Camaro or Firebird, was that the one you used? If I can, I'd rather elongate the holes in the crossmember rather than drill into the frame, would that be possible?
 

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I used a Camaro piece on my 69 because it was the only one available new at the time--I slotted the holes so it could be used. I've never liked the way it fits, but it's been there for a lot of years without failing.
 

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Yeah, it seems silly to be running a TH400 behind what is basically a stock small block, but in the long run I'll be glad I have it when I want to put more power down. The reputation the TH400 has for excellent reliability and design is rather appealing as well.
I've come across a few of those ebay crossmembers, listed for a 1967-69 Camaro or Firebird, was that the one you used? If I can, I'd rather elongate the holes in the crossmember rather than drill into the frame, would that be possible?
Elongating the hole(s) would still not do anything as the hole for the crossmember that needs to be drilled wont be over or near the hole in the frame, the ones listed for the 67-69 firebird are for the BB and are offset, you want the one thats listed for the SB
 

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Elongating the hole(s) would still not do anything
I had to elongate the holes on each end of the crossmember--actually I sawed one end into open slots to make it reach. Like I said, I've never liked the way it fits, but it's never failed.
One of my friends made his from 1-1/2" square tubing that sat in U-shaped channels on each frame rail, with a pit pin to hold it in place.
 
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