[Robert_Z_]

I have a 79 Trans Am that originally had a 403. I purchased a 455 and th400 transmission from a 74 Trans Am. I'm wanting to know what all I would need to change out in order to drop the new engine and transmission in?

[PAUL K]

Motor mount location needs to be changed. Radiator hoses, trans crossmember, driveshaft, throttle cable bracket, shifter cable bracket and all the accessory brackets like the alternator, p.s. pump and pulleys for starters.

[HoneyHush]

The frame mounts should be the same and in the same location. The only thing you need are the correct engine side brackets for a firebird. Easy swap really.

[25stevem]

Also the TB400 takes a different trans yoke if you had a TB350 with the 403 and the TB400 may be longer so you also may need to get the shaft shortened.

There may also be a height difference in the hood shaker base between both motors, I forget.

[jhein]

Sounds like a fun project.

[HoneyHush]

I don’t think the height of the shaker base is different but the shaker plate itself is. The 400 pan has the cut out locate on the base farther forward I think it is.

[HoneyHush]

I’m pretty sure it was Summit or Jeggs that I got my th400 crossmember from and it was only like $50.

[Skip Fix]

The factory type cross members available are for the earlier second gens-different subframes . You will have to use a G Force/Summit or a Bowtie Overdrives 200R one.
https://www.summitracing.com/parts/s...bird/year/1979

[Goatracer1]

I know some of the Olds engines have the motor mounts located further forward on the block than the Pontiac engine. You would have to unbolt the frame mounts and move them. The holes are probably already there to bolt them to.

[grandam1979]

I would just use the 350 trans that’s in the car now. I did a 80 that was a 305 car it’s not that bad to do. Trans was located in the same spot 305 and 455. Best part is your speedometer will still be correct. Good luck with swap

[Half-Inch Stud]

The swap will be wooooorth it. Could sell the 403 to a TA owner and consider the money receive more than offsetting the value difference

[HoneyHush]

My new th400 crossmember was the right length. Didn’t have any problem.

[HoneyHush]

The frames are the same dimensions. They just have subtle differences such as control arm bushing sizes, wheel bearings etc, and the rear mount locations. Here are some things from guys I quickly searched on this py site

[Skip Fix]

The last picture clearly shows why the early TH400 mount will not work on the later frame-holes further up on75-81 and there were no factory second gen TH400 75 and up. You can cut the tab off the late TH350 and weld it back on the rear if you do not have a deep aluminum pan on the TH400. I even made a jig to do it based on the BowTie Overdrive one.
http://forums.maxperformanceinc.com/...ighlight=TH400

[HoneyHush]

Obviously there weren’t any factory installed th400 cars in 79. That cm is the factory part for the car. The holes aren’t there because the factory didn’t install them. Which is easier…drilling four holes or modifying the th350 cm? And then modifying it even more with a deep pan? Honestly this is why people don’t share information on here. I have a 1979 Trans Am that was factory 403 that I just made all these changes to. It’s simple.

[25stevem]

If you will never go racing with slicks then keep the TB350 to get the car back on the road and then build it stronger when you get the chance.
I would put a shift kit in it now and a auxiliary trans cooler.

[Skip Fix]

Originally Posted by HoneyHush

Obviously there weren’t any factory installed th400 cars in 79. That cm is the factory part for the car. The holes aren’t there because the factory didn’t install them. Which is easier…drilling four holes or modifying the th350 cm? And then modifying it even more with a deep pan? Honestly this is why people don’t share information on here. I have a 1979 Trans Am that was factory 403 that I just made all these changes to. It’s simple.

Or buying a new crossmember is equally easy.Just giving all the options, drilling new holes is one but not a "drop on" crossmember. Seems like sharing information on all the options to me. Locating the top hole todrill can be a little difficult also from underneath.

[PAUL K]

A lot of guessing going on in this thread.... Just saying

[sdbob]

I like Transgo shift kits. In the 70s they wrte were great. I hope quality hasn't changed.

[78w72]

the easiest & cheapest option to use a factory TH400 (TB400?) cross member on a later 2nd gen is to just drill a couple new holes in the frame to mount it just like the factory did, no need to cut & weld or buy an aftermarket x-member. i did this on a 1978 t/a that had a 1974 455/TH400 put in it, very simple to do.