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58Bonne's 1958 Bonneville

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350 Engine Swap to a 400

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Considering replacing a '76 Pontiac 350 engine with a 400 (for more grunt). Now attached to a 200 4R tranny (currently could use new converter as lockup is slipping when "warm").   Current stock engine (dog) and tranny. What are the fitment issues (motor mounts?, engine length to fit exist tranny?,  accessory brackets?, exhaust manifolds interchange?, etc?).  Would I likely be better off just swapping-in a warmed-up 350.  Want to stay with Pontiac.  $ budget rather limited.  I should likely just crack open a jug of Dickel, take multiple aspirin, lay down & get over it!!

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You're in luck 58Bonne. Pontiac engines are externally the same size, all they did was vary the bore and stroke of the engine to get the different displacements. So going with the 400 is a matter of taking all your bracketry off your 350 and bolting onto your 400. It will fit. Motor mounts, ditto. The only manifold issues would be if you're new motor had different head port shapes than your old (e.g. "D" port vs round), otherwise everything should swap over. The only trip up point (albeit minor) is the motor mounts. Older 400s had a 3-bolt motor mount pad machined into the block, later 70 400s had only 2-bolts. However Pontiac left the pad for the third hole if you need it. You just need to drill and tap it - if you need it. So cash wise, this is about as cheap as it gets.

The 200 4R tranny will bolt up, but it sounds like you need a new converter anyway.

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Frosty, thx for the prompt reply.  That sounds like the good news I was looking for!  Assuming I now have two-bolt motor mounts,  do you know what years the Poncho 400s also have the 2-bolt motor mounts?  [I would certainly like to avoid any block-drilling.]  Also, do you (or anyone else on this board) have any insight as to what port shape heads I would have on my existing '76 350 and what year 400s would have the other port-shape you referenced?


Thx again for your help...

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I think this chart will answer your question concerning the block.

As for your heads, you would have to tell us what casting numbers you have for your 350 and 400 engines or physically look at the exhaust ports.




Edited by Frosty
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THANKS for chart/table/pix. OK, the plot thickens---my existing '76 350 should have the 5 mount holes (presumably all drilled?)--but I'm now unable to verify such, as I cannot get/see under car. It also looks like I now have some kind of custom mount(s)--as this engine is now in my 58 Bonne.  I don't know whether 2 or 3  mounting holes are now used... I do not yet have a 400 engine, and was now seeking some heads-up info as to what year(s)  engine I should look for to facilitate such a swap.

As I don't have the car at my house, I'll try to check head Casting No. the next time I can get to car and sneak a peek!

THX again,,,

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Happy to help. Just let us know how things go. We'll be here.

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