A question which I cannot answer as my understanding of current rules is insufficient, but may be important - is this a modification which would alter the historic status of the vehicle?
As I understand it, the points system still applies for historic registration. So far as I can see, you would definitely lose the points for the gearbox and may already have lost the point for the engine. That means you cannot have anything else modified/replaced and still retain enough points. The chassis would be such an item.
The historic status for MOT exemption/tax appears to impose quite strict criteria on modifications to the chassis.
The key question is therefore whether moving the engine mounts to shift the engine forward in a SWB would constitute a major modification of the chassis? I am aware that this was allowed for historic (30+ years ago) conversions, but what the consequences of doing it now would be I am not sure.
Something I would be inclined to seek advice on if this is important to you.
Alec