I don't think your plan to increase the length of the mast is viable. It is not just a matter of slapping on some additional aluminum.
Ventures were not exactly overbuilt, so I think the mast you have is probably strong enough. Most early Ventures used the same mast section, 3" X 4", similar to the Dwyer DM-6 (
http://www.dwyermast.com/). Compare what you have to this.
Have you raised the mast on the boat yet? Since it was made for another boat you will probably have to adjust/change the standing rigging. Earlier Venture 21s were very simple. They had no spreaders, just 2 shrouds and a forestay. Later models used a more conventional rig with spreaders and upper and lower shrouds.
According to this site
Sails by Boat the luff of your main should be 21'10" and the foot should be 9'9" long. You need either to have your sail recut or look for a different one.
I see that you have a sliding gooseneck. A tackle attached to this can be used to tension the luff of the main. But you need room for it to slide.
Very few sailors reef their jibs nowadays. Most have roller furling, and those that don't just change down to a smaller jib. (I have reef points on my staysail and use them only once or twice a year.)
An efficient reefing system for the main is essential. Don't venture out for anything but a short day sail without one. I recommend two reefs. Your boat is small enough that a single line reefing system could be used for each reef. Check the Harken web site for how this works. (You can cobble together your own system for about 1/3 of what Harken wants for theirs.)
As you progress in getting your boat ready to sail you will discover that what seems good on paper does not always work out so well in the real world. You may have to change the location of a fitting or discard an idea completely. It's all part of the learning curve. There is no substitute for experience. Get out there and do it!