I believe he is running a 4 speed with 2.64 first gear...unless I missed something. With a 400, yes, with the bore/stroke and 3.55 gears and a RAIII cam, definitely if you have the compression. By the way, what transmission are you running?

However, 55 more cubes, more low end torque, 3.55 is too steep a gear for a 455 with an M20.

If you are daily driving...personally, a 3.08 with a 400 and #62 heads (78-80cc)

Here is my thinking...If you use 2.56 rear gear with a 455 and a daily driver and those heads, you will be sucking fuel down so bad...because the setup is mismatched and inefficient. If your concern is daily driving, why the 455? why such a radical cam? Why a mismatched rear for a torque monster? You will not be close to getting the full potential out of that 455, those heads, that cam and rear..you are probably looking at 8-10 mpg, crappy low end, idle issues..and no performance at all. I say you would be in the 180-200 HP range to be honest. a 455 came with 067 grind and similar cc heads and was pushing only 200 hp. Now you are going to increase cam size by leaps...

An excerpt...from 1975..regarding the 455.

"Rear axle selections were becoming increasingly limited. Automatic/400 equipped Trans Ams had a 2.56:1 rear axle ratio . A 3.08 gear was included when the 4-speed was specified. Included as part of the "Performance Package" in which the 455 V8 was the major element, the ratio was a more satisfying 3.23:1

And what makes the setup even worse so to speak...the heads have "X" amount of porting. Do you have the flow numbers?....more flow, less restriction. That fact adds to the "fall flat on your face" effect.

You need 9 to 9.7 to 1
You need a smaller 067 grind. (IMO an 068 may be too harsh for a "daily driver")
You need gearing in the 3.08 (preferred) to 3.23 arena to bring the potential of the 455 inline.

Based on bore/stroke differential between the 400 and 455 and where the power bands are at...the 400 will wind up faster than the 455.

Honestly, I would confer with Butler on this...

HP CI TR BC CR CAM HC Carb My notes...

360 455 M WA 10.25 68 64 4 70 (068 grind 64 87 cc heads)
370 455 M WG 10.25 68 64 4 70 (same as above)
370 455 A XF 10.25 67 64 4 70 (smaller cam)
360 455 A YA 10.25 67 64 4 70 (smaller cam)
360 455 A YC 10.25 67 64 4 70 (smaller cam)
360 455 A YH 10 67 15 4 70 (small valve heads (1.96/1.66)
335 455 M WC 8.4 68 197 4 71 (Round port heads
335 455 M WC 8.4 68 197 4 71 (round port heads)
280 455 M WG 8.1 66 98 4 71 (smaller cam, small valve 1.96/1.66 114 cc heads)
325 455 M WJ 8.1 67 66 4 71 (same cam as above, large valve same 114 cc heads)
335 455 M WL 8.4 68 197 4 71 (round port, RAIII cam)
335 455 M WL 8.4 68 197 4 71 (same as above)
325 455 A YC 8.1 67 66 4 71
325 455 A YC 8.1 67 66 4 71
325 455 A YC 8.1 67 66 4 71
325 455 A YC 8.1 67 66 4 71
335 455 A YE 8.4 68 197 4 71
335 455 A YE 8.4 68 197 4 71
280 455 A YG, YA 8.1 66 98 4 71
300 455 M WD 8.4 68 7F6 4 72 (round port 111 CC)
300 455 M WM 8.4 68 7F6 4 72
250 455 A YA 8.1 67 7M5 4 72
300 455 A YB 8.4 68 7F6 4 72
250 455 A YC 8.1 67 7M5 4 72
300 455 A YE 8.4 68 7F6 4 72
200 455 A YH 8.1 66 7L4 2 72
250 455 A ZH 8.1 67 7M5 4 72
250 455 M WT 8 67 4X 4 73
250 455 M WW 8 67 4X 4 73
250 455 A YA,XL 8 67 4X 4 73
250 455 A YC,XE 8 67 4X 4 73
250 455 A YD,XM 8 67 4X 4 73
250 455 A YK,X7 8 67 4X 4 73
250 455 A ZC,ZA 8 67 4X 4 73
250 455 A A4 8 67 4X 4 74
250 455 A AU 8 67 4X 4 74
250 455 A UX 8 67 4X 4 74
290 455(SD) M W8 8.4 323 16 4 74
250 455 A Y4 8 67 4X 4 74
250 455 A Y6 8 67 4X 4 74
290 455(SD) A Y8 8.4 323 16 4 74
250 455 A Y9 8 67 4X 4 74
250 455 A YR 8 67 4X 4 74
250 455 A YU 8 67 4X 4 74
250 455 A YW 8 67 4X 4 74
250 455 A YY 8 67 4X 4 74
250 455 A Z4 8 67 4X 4 74
250 455 A Z6 8 67 4X 4 74
250 455 A ZU 8 67 4X 4 74
250 455 A ZW 8 67 4X 4 74
250 455 A ZX 8 67 4X 4 74
200 455 M WX 7.6 67 6H 4 75
200 455 A YU 7.6 67 5I 4 75
200 455 A YW 7.6 67 5I 4 75
200 455 A ZU 7.6 67 6S 4 75
200 455 A ZW 7.6 67 6S 4 75
200 455 M WX 7.6 67 6H 4 76
200 455 A Y3 7.6 67 6H 4 76
200 455 A Y4 7.6 67 6H 4 76
200 455 A Y8 7.6 67 6H 4 76
200 455 A Z3 7.6 67 6S 4 76
200 455 A Z4 7.6 67 6S 4 76
200 455 A ZB 7.6 67 6S 4 76

Well, you get the hint....