455 in 78 GMC

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LARRYT

Member
Feb 9, 2020
74
3
8
GMC Fans .Soo here i sit after buying 78 gmc 455 (verified by 2 former owners) waiting to go get it sometime late in march ( sight unseen; oh! oh! April fools day )
OK now ,,how can i tell by looking. to verify such claims
ps it a honor having you all to consult thanks
Larry 455-78
 

Christo

Administrator
Staff member
Oct 4, 2019
279
74
28
Weymouth, MA
I saw a thread recently that said the easiest way to tell 455 vs 403 was the number of bolts holding down the valve covers. 403 has fewer. Can someone help me out with the specifics?
 

LARRYT

Member
Feb 9, 2020
74
3
8
by the way just happen to have a photo i can see 3 and there must be 2 more by the spacing . 455 confirmed happy now Larry 78 /455
 

LARRYT

Member
Feb 9, 2020
74
3
8
great photo indeed !tried to send a( bit mat photo) system would not take it thanks gents larry
 

Kevin F

New member
Oct 9, 2019
13
4
3
Ive been thinking of having our 77 royale 403 with 120,000 miles rebuilt, but have a known 455 available that can also be rebuilt. Which is a better option as far as power or preference. Also can everything from the 403 be used on the 455? obviously not the intake but most everything else? Not sure how different the 455 block is vs the 403..........:unsure: Thanx guys
 

Matt Colie

Active member
Oct 25, 2019
384
95
28
South East Michigan near DTW
As I am currently sitting on both, I can pretty much assure you that there is very little that is common between the two. If I was to make a list, it would be that both bolt to a TH425, they use gasoline and have 12V electrics, and I think the mounts are the same, but I know that the FEAD brackets, exhaust, and the coolant hoses are different. Besides, you add about 125# for what? As I recall, about 20Hp? Ask the people over at the GMCsite before you do any change.

Matt
 

Hal

Member
Oct 3, 2019
40
25
18
71
I think the 403 is really the better choice although I'm sure some won't agree. The difference in performance is really a push with the right gearing and the 403 is capable of nearly 2 mpg better mileage. The difference between a big block and a small block as far as weight. Just my experience with two 1977 Royales', one a 403, the other a 455. 8 mpg vs 10 mpg.
Hal