The most practical way price wise I would install a 4.6 motor.
I'm also in Cali with a 95 D1 5-speed powered by a 4.6 with Piper cam, Rpi chip, high flow cats and Borla exhaust. Intake manifold and heads port matched as well the iron exhaust manifolds at the heads and the exit end opened up as well the beginning of the down pipes. Few other control adjustments like cold ram air, throttle position sensor, restricted hot water to intake plenumn and adjustable restriction for IAC valve, cold starts idle at 550 rpm's after the electronic pre-oiler delay timer has lubed and LED turns green to start. Lucky we are full time 4 x 4 so the smog sniff test only and no dyno.
With the new cats I had 4 of 100 (max) HC at idle, 5 of 180 (max) HC at 2,500, the numbers went up to 6 and 7 two years later. I could of installed a hotter cam but then I don't want drive train reliability problems and it runs a hell of a lot better than the 3.9 and still pull 600 rpm's in 3rd around town.
At the minimum a 4.6 should of been installed in D1's back in 94 as 4,600 pounds with a 3.9 is a joke even on the flat not pulling a grade on the highway, wait and pull a small trailer up a grade. Sure many claim they pull a 4,000 pound trailer or boat with ease, so will a old 40 hp VW and become a road hazard. Not to start a ******* forumn war, just what I did to improve my D1, if Rover didn't have the 5-speed I would of looked at different brands as auto suck in my book as well that roto crap joint. Just installed a real driveshaft, again JMO's. BTW Rover no longer sells 3.9, 4.0 or 4.6 short or long blocks, you'll have to buy used. My replacement was under a 7 year 100K mile warranty thing coverage and the 4.6 short block was cheaper ( better and cross bolted mains) than another 3.9 block. Check into Rimmer Brothers or RPI. I have found a 330ci Buick crank will fit a block as you can always use a few more inches (she said).
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