Vintage race engine (273 vs. Slant 6)

I'm planing on doing some vintage racing with my 65 Valiant.
The classes are limited to engines and speed parts from the era so I'm trying to decide between a 273 or a Slant 6.

Below are the allowed modifications.
I'd like to hear your suggestions.


6 Cylinder

Allowed modifications:
Those ones considered as from the “period” (if there exists proof or evidence) and approved by
scrutineering.
To improve the ignition system.
The use of alternators instead of generators.
The uses of tube exhaust headers.

8 Cylinder

Allowed modifications:
Those ones considered as from the “period” (if there exists proof or evidence) and approved by
scrutineering, in all V8 engines powered vehicles which original weight is below 3,200 lb. (1,452
Kg.).
The maximum intake permitted will be one carburettor with 2 barrel of 500 cfm each.
A maximum overbore of 0.040” (over original displacement of the model available in 1965).
To improve the ignition system.
The use of alternators instead of generators.
The use of modern tires of maximum (no lower number) aspect ratio 55, 60, 65 or 70 (only if they
are fitted in the original wheel diameter and 6 inches width maximum wheels).
The uses of tube exhaust headers.