take it to the next level?

I'm curious about upgrading my 750 rh to the next level so a few questions I'd like to toss out are about modernizing my 81 seca. I'm curious wheather it would be easier to fillet my drive shaft cover or fabricate some sort of strut arm in order to install a wider tire? I shortened the seat (ala, xj750p) and the cheezy plastic sub fender, splash guard between the tire and the air box shows, is there a most likely (to fit) metal replacement available $200 for something that looks like something from a Schwinn is absurd. Will the rear hub from a 650 v-star swap in if I wanted to go a 15" wire wheel in the rear?

I fabricated a ram air system that works well for power and response at highway speed but when I ride at sustained speeds over 80 mph, it tends to regurgitate motor oil through the air box. In fact the other day I down shifted into 4th over 90 and oiled down my rear tire and got a little loose. I'd prefer a closed system, is there a check valve like from a turbo system that can eliminate this?
The prototype H.D. 48 looks like what I've been working towards chassis wise but I love the scream of my xj.

 

downshifted at full throttle that is.

 

have you considered upgrading to a newer motorcycle

 

yes. but I want basically the same thing that I have, or am making. A high performance inline 4 bobber style cruiser, sort of a retro 70/s style drag bike.
I don't want a bike any heavier than my xj750, and I'd rather lose another 50 pounds (the bike too). The current crop of bikes pale in comparison performance wise below $7000 which is way out of my price range. I don't like odd fire v-twins like harley and honda and just the look of a brace of four pipes sticking out of the front of the head and winding down beneath does something to me like a supercharger sticking through the hood of a nova or a falcon.
I have considered going to a Kawi 500 ltd with a ninja top end and full race carbs. But that's a bike for a different forum.