Turbo Phil or one of the other guys would probably be able to better advise here, however, given that you're limited to the twin 1-1/4" SUs, that really
limits you to circa 100 HP unless you can modify the carbs, but even then, it won't add much more to the potential of these Carbs, maybe add another 5 HP at best.
Given your criteria here, being that you are looking for overall improvement, but a better bottom end and an upper rev limit of 7000 RPM, I don't feel that going to such big valves will be helpful here, in fact, likely to opposite, however, your choice of 1.44" Inlets would likely be the way forward. I don't think going any bigger would be helpful. It would be a good idea to off-set the guide(s) if for no other reason than to improve the reliability of the casting (ie, less likely to crack).
Gotya on what you are meaning with the valve centres vs the bore centre. They are not equally divided from the bore centre, they are off-set towards the exhaust as that is the smaller valve, just exactly what they are, I don't have to hand, but in any case, do vary from engine and head combinations. On that' if you are going to these bigger valves, I'd suggest a dry assembly of the head to the block and checking, as they may also not be over the bore centre line either. If they are out, this could (and should) be corrected to take better advantage of the head you are preparing.