:welcome: to the forum.
There is a timing window on the left side of the block. The timing marks on the flywheel, you may need to clean them up to see them. Timing needs to be set 12*BTDC. Max advance is 24-28*BTDC.
I do not recommend modifying your lift arms to work with your box blade. They will not work right for other attachments then and it will cut down on your lifting capacity by extending them. You would also need a new toplink. I think the best thing would be to raise the attachment points on the box blade.