Pontiac Owners Club UK

I have a 1969 Firebird 6 cylinder with 2 speed auto trans which I am toying with swapping out for a V8. i have been offered a complete carb to pan 400 engine and TH400 box and wondered whether anyone has any experience with this swap? I have done a quick sweep of the forum and couldn't see a similar thread but apologies if I have missed something.

I haven't attempted anything on this scale before but am quite competent mechanically and have help available if I need it. The obvious things/questions that spring to my mind are:

1. Engine mounts - presumably I will have to fabricate the engine or body side or both?
2. Transmission mount - as above?
3. Would I need to shorten the prop shaft for the TH400 and/or modify the box end of the shaft?
4. Bigger radiator, which will require some fab to mount presumably?
5. Uprated front springs - does anyone know if 66 GTO front springs are compatable? I have been offered some you see.
6. Power steering pump - not sure if one is fitted to new engine but may require some fiddling?
7. Exhaust system - i will need to fab this.
8. Shifter - will I need to change this?
9. Brake upgrade!!!

If anyone can offer any answers to the above, advice, warnings or pointers it would be most appreciated! Many thanks for taking the time to read.

I'm afraid I can't answer all of your questions but I can help with a few.

The engine is mounted with only 2 mounts onto the cross member which goes underneath the engine and then Supports the front hub and suspension. The mounts are usually bolted into place, some are welded. It's easy to swap them either way and 400 mounts are readily available at many shops. You will most likely have to measure and re-drill the mount holes. The only other engine support is via the transmission and it's mounts.

Radiator supports are usually standard across all engine models in the 2nd gens I've seen - I would imagine 1st gen is the same. A larger rad will simply sit on the rubber mounts as the old one did but you may require a larger fan shroud. Not sure on that.

As for exhausts well they are readily available to buy off the shelf or get a custom stainless one built. Headers are cheap and you won't have to modify the underside of the car to suit a dual v8 exhaust system. The mounts / hangers are at the rear of the car as standard (usually above the axle and then 2 for the tail pipe section just above the rear end of the leaf spring either side. They are not like modern hangers - more of a canvass strip belt and clamp around the pipe above the axle and a solid metal bracket which slides into a metal slot with rubber to dampen the vibration.

I'll leave the rest to the others !

rad is the same size, just double row cored so fits ok
crossmember bolts in same place, just get the one for the box
prop for the matching car/box configuration should fit ok
a lot of it will be trial and error, get the engine/box in and try fit things

Thanks Ian! Thanks John! That's very useful information, I appreciate the help.

hi there it should be pretty easy to swap you have to think it would just be a bolt in bolt out job as of coarse the v8 was an option from new so they would of not made it hard for the factory to install any of the engine options ,
1,engine mounts just get for the 400 your putting in the 350,400,455 all fit on the subframe
2,transmission again just get the mounts for the gear box your installing
3,just get the prop for your box ( what rear axle is fitted to yours)
4,rad should just slot in no problem may well need top and bottom hoses as long as your rad has side oil coolers ?
5,springs im not sure on just put the engine in and see how the ride height is
6,pas pump just get the one for the engine along with hydrolic lines if needed
7,exhuast you will need a new one along with headers or down pipes
8,shifter will be wrong as your new box will have 3 gears not 2
9,brakes depending on what rear axle you have as long as you have discs on the front you should be fine or you could upgrade to a larger master cylinder
so as you can see should be just a matter getting all the parts together and fitting
good luck